Intrauterine period of fetal life. The main stages of development. Critical periods of development. Critical periods of intrauterine development

22.06.2019

FERTILIZATION

AND DEVELOPMENT OF THE FETAL EGG

ONTOGENESIS

Questions

1. Fertilization and development of the ovum

2. Ontogeny

3. Critical periods of development of the embryo and fetus

Fertilization and development of the fetal egg

After ovulation, the egg first enters the abdominal cavity; and then into the fallopian tube, where fertilization can occur, in order for this to happen, the sperm needs to penetrate inside the female reproductive cell, and this is a kind of fortress. To take it, you need to destroy the shell of the egg. The tools of the spermatozoon are enzymes that break down the substances from which it is built, one spermatozoon cannot cope with it. It can be performed by at least four male sex cells. However, only one of them penetrates the resulting gap, then complex physico-chemical changes occur in the egg cell membrane, and it becomes impregnable for other spermatozoa. After fertilization, the cell first contains two nuclei - the egg and the sperm, but moving towards each other, they finally merge: a unicellular embryo is formed - a zygote with a karyotype of 46 chromosomes that is normal for a person.

Ontogenesis

Since fertilization egg begins the first of three periods of intrauterine development of a person: this period is called blastogenesis (Greek blastоs - sprout, embryo). It lasts 15 days.

Driven by the fringes that cover the fallopian tube from the inside, carried away by the flow of fluid in it, the embryo slowly approaches the uterus. 30 hours after fertilization, the first division (crushing) of the zygote takes place. Then there is one division per day.

By the 4th day when the embryo reaches the uterus, it is a lump of 8-12 cells. For the next 3 days, the embryo floats in the fluid surrounding the uterine mucosa. Here, the cells split up faster, and by the middle of the 6th day, the embryo already consists of more than a hundred cells. At this stage, it is called morula. On its surface, cells divide faster and look lighter. They form a shell - trophoblast. Darker large cells, located under the light ones, form the germinal nodule - the embryoblast.

By the time the fetus enters the uterus, it is already prepared to receive it. under the influence of the corpus luteum hormone progesterone, its mucous membrane thickens 3-4 times, swells, becomes loose. Many additional blood vessels develop in it, glands grow.

By the 7th day after fertilization, the embryo changes its structure again. Now it is no longer a bunch of cells, but a blastocyst vesicle. The trophoblast forms its surface, and the embryoblast moves from the center of the vesicle cavity to the side. The embryo is ready for implantation into the uterine mucosa - for implantation. Its superficial cells begin to secrete enzymes that destroy it. Outgrowths appear on the trophoblast, they are fast. enlarge and grow into the uterine tissue. The blood vessels are destroyed, and the embryo is immersed in the outflowing blood. Now this is the environment from which he will draw nutrients and oxygen until the placenta is formed. It takes 40 hours for the embryo to implant.

In the next few days, two vesicles form in the embryo - vitelline and amniotic (from which the fetal bladder will develop in the future). In the place of their contact, a two-layer germinal shield appears. The "roof" of the yolk sac is its lower layer (endoderm), and the "bottom" of the amniotic sac is its upper layer (ectoderm). By the end of the 2nd week, the back of the embryo thickens - axial organs begin to form in it. During this period, the nutrition of the embryo is autonomous, due to the yolk sac - the yolk type.

From the 16th day the second, or actually embryonic "period of intrauterine development of the child begins, which ends by the 13th week. Changes in the embryo grow like an avalanche, but following a clear plan, here is a brief chronology of events.

During the 3rd week Between the ecto- and endoderm, another layer is formed - the mesoderm. These three germ layers - e-ectoderm, mesoderm and endoderm - will later give rise to embryonic rudiments, from which all the tissues and organs of the child will develop. By the end of the week, the neural tube is visible in the ectoderm, and below, in the mesoderm, the dorsal string. At the same time, the heart tube is laid, a stalk is formed - a cord (allantois), connecting the embryo with the villi of the chorion - the embryonic membrane formed from the trophoblast. The umbilical vessels pass through the allantois (abdominal stalk) - this allantoic nutrition.

On the 4th week many organs and tissues of the embryo are laid: the primary intestine, the rudiments of the kidneys, bones and cartilage of the axial skeleton, striated muscles and skin, neck, eyes, thyroid, gland, pharynx, liver. The structure of the heart and the neural tube, especially its anterior part, the future brain, is becoming more complicated.

On the 5th week the length of the embryo is 7.5 mm. At the age of 31-32 days, the rudiments of hands appear, similar to fins. The transverse septum of the heart is laid. In it. time using ultrasound is possible. clearly see the beats of the heart. This means that the fetus already has a circulatory system. The organs of vision and hearing develop, the organs of smell, the rudiments of the tongue, lungs, and pancreas are formed. The renal tubules reach the cloaca, and the rudiments of the ureters of the posterior kidney. Sexual tubercles appear.

6th week marks the beginning of blood circulation in the liver.

By the 40th day leg buds appear.

AT during the 7th week eyelids, fingers, and then toes are laid. The formation of the interventricular septum of the heart ends. The testicles and ovaries are clearly expressed.

AT end of week 8 in an embryo 3 ": 3.5 cm long, the head, torso, rudiments of limbs, eyes, nose and mouth are already clearly visible. By the microscopic structure of the gonad, one can determine who will be born - a boy" or a girl. The fetus is in the amniotic sac filled with amniotic fluid.

On the 3rd month in the embryo, the cerebral cortex is clearly visible. By week 12 hematopoiesis is formed in the bone marrow, leukocytes appear in the blood, and by the end of this week - hemoglobin, as in an adult, the formation of blood group systems occurs.

From the 13th week the third, or fetal (fetal), period of intrauterine development of the child begins.

By this time, the period of fetal organogenesis and the formation of the placenta is completed .. The embryo is surrounded by amniotic fluid and three placental membranes, two of which are fetal (amnion and chorion) and one is maternal - decidual from the functional layer of the uterine mucosa. The fetus connects to the placenta with a cord-like formation - the umbilical cord, in which two arteries and one vein pass. The vessels are surrounded by a specific tissue - Wharton's jelly. Nutrition becomes placental.

Amniotic fluid is a complex biologically active medium involved in the exchange between mother and fetus.

The waters contain protein, hormones, enzymes, macro- and microelements, carbohydrates and other substances. By the end of pregnancy amniotic fluid 1-1.5 liters. Many substances found in the amniotic fluid can affect the fetus.

From the 14th week the fetus is already moving, but the mother does not notice these movements yet.

At 16 weeks the weight of the fetus is approximately 120 g, and its length is 16 cm.

His face is almost formed, the skin is thin, but there is no subcutaneous fat yet. Since during this period the muscular system develops intensively, the motor activity of the fetus increases. Weak respiratory movements are noted. It has also been found that the fetus is aged 16.5 weeks, if you touch his lips, opens and closes his mouth, 18 week fetus in response to irritation of the tongue, initial sucking movements are observed. During 21-24 weeks the sucking reaction is fully developed. The woman feels the baby stir for the first time between 16th and 20th weeks. By the end of the 5th month you can already count up to 2000 fetal movements per day. Its length at this time reaches 25 cm, and its weight is 300 g. The doctor can already listen to the child's heartbeat.

The skin of the fetus, starting from the head and face, is covered with the finest hairs (fluff). Meconium (original feces) is formed in the intestines. The formation of subcutaneous fat begins.

At the end of the 24th week the length of the fetus is already about 30 cm, and its weight is approximately 700 g. Its internal organs are formed so that in the event of premature birth, such a child can live and develop in special conditions,

At the end of the 28th week pregnancy, the length of the fetus reaches 35 cm, and its weight is 1000 g. Its entire body is covered with fluff, the cartilages of the auricles are very soft, the nails do not reach the fingertips. The skin of the fetus begins to be covered with a special generic lubricant that protects it from soaking (maceration) and facilitates the passage of the fetus through the birth canal. He becomes very active, and the mother feels his movements constantly, as he is still moving freely in the fetal bladder. The position of the child is still unstable, the head is usually directed upwards.

By the end of the 32nd week the fetus has a length of about 40 cm, and a weight of 1600 g, at the age of 38 weeks - about 45 cm and 2500 g.

By week 40 the fetus is quite ready for existence outside the mother's body. The length of his body is on average 50-51 cm, weight - 3200-3400g. Now the child, as a rule, is located head down. His position becomes stable, because due to the large size of the body, he cannot move freely in the mother's womb.

CRITICAL PERIODS OF EMBRYO AND FETUS DEVELOPMENT

Intrauterine development, starting from fertilization, lasts 266 days (or 280 days from the l-th day of the last menstruation) and has two periods - embryonic and fetal,

1. Embryonic period

1) Lasts three weeks. It is characterized by the transformation of the egg into a small embryo, which is introduced into the uterine mucosa.

2) Continues until the 10th week of pregnancy. There is a crushing of germ cells, their migration and differentiation into various specific organs. At the end of the 8th week, the main organ structures are differentiated, but the functional development of the organs is not completed - during this period, gross defects may occur.

2. Fetal period

From 10 weeks of pregnancy to childbirth, growth processes predominate. The most important moments of this period are the formation of the palate, the differentiation of the external genital organs and the histogenesis of the CNS.

In the fetal period, harmful factors do not cause the formation of morphological defects, but can cause various kinds of behavioral disorders or mental development disorders in the postnatal period.

Under the critical periods of development understand the moments of development, which are characterized by a slowdown in the process of growth of structures and a decrease to the minimum values of redundant information in biological systems, leading to the formation of a new quality. Knowledge of critical periods of development is the basis for understanding the effect of the environment on ontogeny and pathogenesis. embryopathy, fetopathy including hereditary diseases and malformations.

There are the following critical periods of development:

1. for the whole organism - harmful factors can lead to the death of the embryo. Thus, according to WHO, during a normal pregnancy, 300 fetuses out of 1000 pregnancies die.

2. Private critical periods - exist in the ontogeny of each organ, associated with the non-simultaneous initiation and rate of differentiation of organs and systems.

3. Critical periods of cell development as a biological system. There is evidence of critical periods in the development of individual cell organelles.

l-th critical period from 0 to 10 days - there is no connection with the mother's organism, the embryo either dies or develops (the principle of "all or nothing").

2nd critical period from 10 days to 12 weeks, the formation of organs and systems occurs, the occurrence of multiple malformations is characteristic. The duration of exposure to an adverse factor is important.

3rd critical period (within the 2nd) 3-4 weeks - the beginning of the formation of the placenta and chorion. Violation of its development leads to placental insufficiency and, as a result, to the death of the embryo or the development of fetal hypotrophy.

« Critical periods of embryonic development

mammals and humans»

Students Ogarkova Maria Andreevna

Specialties 060601

"medical biochemistry"

Well 1

Group 2

Head: Nikonova Yulia Mikhailovna

Arkhangelsk

Introduction 3

GROWTH AND MATURATION OF THE FRUIT 4

EMBRYO PERIOD 4

FRUIT PERIOD 4

FIRST CRITICAL PERIOD 5

SECOND CRITICAL PERIOD 6

THIRD CRITICAL PERIOD 8

Conclusion 11

literature 12

Introduction

At present, it is firmly established that the pathology of a newborn is often due to adverse effects during its intrauterine development.

It is known that individual tissues and organs are formed during different periods of embryonic and fetal growth. At the same time, the tissues of the body at the moment of maximum intensity of the processes of differentiation become highly sensitive to the damaging effects of the external environment (ionizing radiation, infections, chemical agents). Such periods, which are characterized by increased sensitivity to the effects of damaging factors, are called "critical periods of embryogenesis". The probability of formation of deviations in development during critical periods is the highest.

Thus, knowledge about the stages of development and growth of the fetus is necessary in order to recognize and avoid possible complications that may occur during pregnancy.

Growth and maturation of the fetus

Pregnancy lasts an average of 280 days, or 10 obstetric months from the first day of the last menstruation (duration of an obstetric month is 28 days; 10 obstetric months equals 40 weeks). During this time, a mature fetus capable of extrauterine existence develops from a fertilized egg.

In the intrauterine development of a person, two periods are (conditionally) distinguished: the embryonic (embryonic) and fruiting.

Embryonic period

It continues from the moment of fertilization until the end of the second month of pregnancy. During this period, the beginnings of all the most important organs and systems (nervous, hematopoietic, cardiovascular, digestive, excretory, etc.) are formed; there is a formation of the trunk, head, face, rudiments of limbs. The embryo acquires the features characteristic of a person. Development processes during this period are very intense, adaptive mechanisms are not yet developed, therefore the embryo is very sensitive to the action of damaging factors.

fruiting period

It starts from the end of the second - the beginning of the third month of pregnancy and continues until the birth of the child. During this period, the fetus grows rapidly, the development of tissues, organs and systems that are in their infancy, the formation of new functional systems that ensure the vital activity of the fetus during its intrauterine life and after birth.

The development of the fetus occurs in direct relationship with the mother's body, which involves biochemical, immune, endocrine, nervous and other mechanisms.

A woman's body during pregnancy is under the influence of a variety of environmental factors that affect the course of pregnancy and in some cases cause the development of a particular fetal pathology. However, the reaction of the fetus to damaging factors depends on the stage of its development and the sensitivity of its tissues to their effects.

In the embryonic and fetal periods, it is customary to distinguish stages when the embryo is highly sensitive to damaging substances. These periods are called critical. The selection of these periods is very important. Critical periods of development are characterized by an increase in metabolic processes (during these periods, all the necessary conditions are created for the implementation of one of the stages of development of the embryo as a whole, as well as individual rudiments of organs and even cells).

In human development, P. G. Svetlov emphasizes the great importance of the following critical periods: implantation, placentation and perinatal (birth).

There are the following periods of fetal development during pregnancy:
pre-implantation(from the moment of fertilization of the egg with sperm until the introduction of the fertilized egg into the mucosa of the uterine wall);
implantation(attachment of a fertilized egg to the wall of the uterus);
organogenesis and placentation(the period of formation of all organs and tissues of the fetus, as well as the placenta);
fetal- the period of growth and development of formed organs and tissues.

Pre-implantation period

Normally, 12-14 days before the expected menstruation, ovulation occurs, that is, an egg that has reached a large size leaves the ovary, enters the fallopian tube, where fertilization most often occurs. From this point on, pregnancy begins. A fertilized egg continues its journey through the fallopian tube for 4 days towards the uterine cavity, which is facilitated by:
contraction of the smooth muscles of the wall of the fallopian tube. These contractions normally occur in a unilateral direction - towards the uterine cavity from the end of the tube facing the abdominal cavity;
movement of the cilia of the mucous membrane, which covers the fallopian tube from the inside. The fluid in the tube begins to move, and with the flow of this fluid, the fertilized egg enters the uterus;
relaxation of the sphincter (circular muscle) in the junction of the fallopian tube with the uterus. This sphincter is designed to prevent a fertilized egg from entering the uterine cavity prematurely, before the uterus is ready to receive a fertilized egg.

The movement of the egg through the fallopian tube occurs under the influence of the female sex hormones estrogen and progesterone. Progesterone is a pregnancy hormone that is produced in the ovary in the early stages of pregnancy (a corpus luteum forms at the site of a burst follicle, which produces this hormone in large quantities and contributes to the onset and maintenance of pregnancy). If progesterone is not produced enough, the egg from the fallopian tube will enter the uterine cavity late. With increased peristalsis of the fallopian tube, the fertilized egg will enter the uterine cavity before it can penetrate the mucous membrane, as a result of which the egg may die. Since in this case the pregnancy will not take place, there will be no delay in the next menstruation, the pregnancy will remain undiagnosed, unrecognized.

The period of advancement of a fertilized egg through the fallopian tube is considered the first critical period of pregnancy (from 12-14 to 10-8 days before the next menstruation). As a result of a violation of the complex mechanisms of regulation of the fallopian tube, the egg after fertilization can also be introduced into the wall of the tube (ectopic pregnancy).

Implantation period

This period also passes even before the expected menstruation, most often when the woman is still unaware of her pregnancy. Once in the uterine cavity, the embryo already consists of 16-32 cells, but it does not immediately penetrate into the uterine mucosa, and is in a free state for another two days. These two days from the moment the fertilized egg enters the uterine cavity to its attachment to the uterine wall constitute the implantation period. The place of implementation depends on a number of circumstances, but most often it is the anterior or posterior wall of the uterus.

The nutrition of the fetal egg during this period occurs due to the local dissolution of the mucous membrane of the uterine wall with the help of enzymes secreted by the fetal egg. After 2 days, the fetal egg is introduced into the uterine mucosa, which contains a large amount of enzymes, glycogen, fats, trace elements, protective antibodies and other biologically active substances necessary for the further growth of the embryo.

The second critical period of pregnancy is implantation, that is, the attachment of the fetal egg to the wall of the uterus. If implantation fails, then the pregnancy ends under the guise of menstruation (in fact, this is an undiagnosed miscarriage at a very short time). Since there is no delay in menstruation, the woman does not even assume that she is pregnant.

The process of implantation is greatly influenced by hormonal factors: the concentration of hormones such as progesterone, estrogens, prolactin (a pituitary hormone - a gland located in the brain), glucocorticoids (adrenal hormones), etc.

Of great importance is the preparedness of the uterine mucosa for implantation, its readiness to accept a fetal egg. After abortion, curettage, long-term wearing of an intrauterine device, infections, inflammatory processes, the receptor (receptive) apparatus of the endometrium may be disrupted, that is, hormone-sensitive cells located in the uterine mucosa do not respond correctly to hormones, due to which the uterine mucosa is not sufficiently prepared for upcoming pregnancy. If the fetal egg is not active enough, does not release the required amount of enzymes that destroy the uterine mucosa in a timely manner, then it can penetrate the uterine wall in the lower segment or in the cervix, resulting in cervical pregnancy or abnormal placentation (the placenta blocks the exit from the uterus partially or completely ).

The presence of adhesions (synechia) in the uterine cavity after inflammatory processes, curettage, as well as uterine fibroids, can also interfere with normal implantation.

Period of organogenesis and placentation

This period lasts from the moment the fetal egg is introduced into the uterine mucosa until 10-12 weeks of gestation, when all the organs and tissues of the fetus, as well as the placenta, are fully formed (children's place is the link between the fetus and the mother's body, through which nutrition processes take place, metabolism and respiration of the fetus in the womb). This is a very important period of intrauterine life, because. at this time, all the organs and tissues of the fetus are laid. Already on the 7th day after the fertilization of the egg, the mother's body receives a signal of pregnancy due to the hormone - chorionic gonadotropin (CG), which is secreted by the fetal egg. CG, in turn, supports the development of the corpus luteum in the ovary. The corpus luteum secretes progesterone and estrogens in sufficient quantities to maintain pregnancy. At the initial stage of pregnancy, before the formation of the placenta, the corpus luteum takes on the function of hormonal support for pregnancy, and if for one reason or another the corpus luteum does not work properly, then there may be a threat of miscarriage, miscarriage or non-developing pregnancy.

The entire period of organogenesis and placentation is also a critical period of intrauterine life of the fetus, because the fetus is highly sensitive to the damaging effects of the environment, especially in the first 3-6 weeks of organogenesis. This critical period in the development of pregnancy is especially important, because. under the influence of adverse environmental factors, the embryo may die or develop abnormalities.

During these periods, the influence of environmental factors on the embryo is especially dangerous, including:
physical (ionizing radiation, mechanical effects); this can be the action of ionizing radiation, for example, in the conditions of a man-made disaster at nuclear facilities, mechanical effects in the form of vibration, and so on. in the relevant industries or at the time of sports training;
chemical: phenols, nitric oxide, pesticides, heavy metals, etc. - these substances can also enter the body of a pregnant woman if she works in the relevant industries or during repairs in a room where a woman stays for a long time. Chemicals include nicotine, alcohol, certain drugs such as those used to treat cancer, etc.;
biological (for example, herpes virus, cytomegalovirus, rubella virus, etc.).

It must be emphasized that during critical periods, harmful effects lead to the most severe consequences - the death of the embryo or the formation of gross malformations.

According to French researchers, if a pregnant woman for the first time in her life encountered cytomegalovirus, a pathogen that causes a disease that in adults can occur as a banal acute respiratory disease (acute respiratory disease) during pregnancy (as seen from a blood test for CMV immunoglobulins), especially on early terms, then in 1/3 of cases fetal malformations may occur. If, before pregnancy, she was already infected, the body turns on defense mechanisms to fight the virus in time, this probability decreases to 1%. The same can be said about the herpes simplex virus.

Of particular danger is the rubella virus when infected with it in the early stages of pregnancy. In such cases, a woman is recommended artificial termination of pregnancy, tk. there is a high risk of having a child with such malformations as microphthalmia - a malformation of the eyes, microcephaly - a serious malformation of the brain; deafness, congenital heart defects, etc.

Of the chemical compounds, lead, mercury, benzene, nicotine, carbon oxides and other substances that can cause malformations have a particularly adverse effect on the state of the embryo.

Some drugs are especially contraindicated during pregnancy (eg anticancer antibiotics); if they were taken, early termination of pregnancy is recommended. When taking certain medicines, it is necessary to consult a geneticist, carefully monitor the condition of the embryo and fetus during pregnancy (ultrasound, blood test for chorionic gonadotropin, alpha-fetoprotein, estriol, which make it possible to suspect the presence of fetal malformations - the analysis is carried out at 16-20 weeks of pregnancy ).

Women working in the chemical industry during pregnancy must be transferred to other, less dangerous workshops. As for the effect of radiation, if it affects a woman before the implantation of the embryo (during the pre-implantation period), in 2/3 of the cases the embryo dies. During the period of organogenesis and placentation, malformations often occur or intrauterine death of the embryo or fetus occurs.

At 7-8 weeks of pregnancy, the reverse development of the corpus luteum in the ovary usually begins: figuratively speaking, the ovaries transfer the function of hormonal support of pregnancy to the chorion (future placenta), and if the chorion is not sufficiently developed, not active, then there is a threat of termination of pregnancy.

7-8 weeks is also a critical period for the development of pregnancy. Very often, a miscarriage, an undeveloped pregnancy or a threat of miscarriage (bloody discharge from the genital tract, pain in the lower abdomen and lower back) appears precisely at this time. If this happens, the woman needs hospitalization. The hospital uses various medications to help keep the pregnancy, if possible.

So, as we have seen, the first trimester of pregnancy almost entirely consists of critical periods, so at this time it is especially important:
if possible, eliminate the negative impact of harmful production;
change the complex of physical exercises during active training in the period before pregnancy, postpone extreme sports for the postpartum period;
spend enough time outdoors;
enough time (8-10 hours) to sleep;
do not take an active part in the repair of premises;
give up bad habits, especially such as drinking alcohol, drugs, smoking.

fertile period

From 12 weeks of pregnancy, the fetal period of intrauterine life begins, which lasts up to 40 weeks. At this time, the fetus is already fully formed, but physically immature.

Pregnancy periods of 13, 20-24 and 28 weeks are critical for patients with hyperandrogenism - an increased content of male sex hormones - due to the onset of fetal hormone production. During these periods, it is necessary to check the level of hormones and adjust the dose of drugs that are prescribed to reduce the amount of male sex hormones (dexametlezone, metip-red, etc.). At the same time, the doctor monitors the condition of the cervix, since an increase in the amount of male sex hormones can lead to its premature opening.

At 13 weeks of pregnancy, the male fetus begins to produce its own testosterone - the male sex hormone, at 20-24 weeks the production of cortisol and male sex hormones by the adrenal cortex of the fetus begins, as a result of which a woman with hyperandrogenism may have another rise in male sex hormones, which will lead to termination of pregnancy.

At 28 weeks, the fetal pituitary gland begins to synthesize a hormone that stimulates the adrenal glands - adrenocorticotropic hormone, resulting in increased production of male sex hormones, which can also lead to termination of pregnancy. If necessary, the doctor will adjust the dose of medications at this time.

So, the action of adverse factors during critical periods of pregnancy can lead to the most adverse consequences. Therefore, a woman during the entire time of waiting for a child, and especially during critical periods, should avoid the action of adverse factors and consult a doctor in case of any “malfunctions”. I would like to advise expectant mothers to take care of themselves, especially since pregnancy lasts only 9 months, and the health and life of your baby depends on its course.

What should be cause for concern?

If the action of adverse factors at a critical time has led to the threat of termination of pregnancy, women complain of pain in the lower abdomen, in the lower back - pulling or cramping. Pain may be accompanied by bloody discharge from the genital tract. Such symptoms should not be left without due attention, because. following them, massive bleeding may occur due to incomplete spontaneous miscarriage, in which the pregnancy cannot be saved.

It is very important at the first symptoms of a threatened miscarriage to immediately contact a gynecologist, undergo the necessary studies, including an examination on the armchair, ultrasound, hormonal blood tests for female sex hormones, male sex hormones, thyroid hormones.

Jasmina Mirzoyan
Obstetrician-gynecologist,
cand. honey. Sciences, Moscow
Article provided by the journal
"9 months", 2006

Normally, a woman carries an unborn child for 40 weeks, which is 10 obstetric months of 28 days. During this period, the embryo makes a long and difficult journey, developing from two parental cells into an independent organism. Among the total duration of pregnancy, experts identified the most responsible and at the same time critical periods of fetal development.

Stages of intrauterine growth

Obstetricians divide the period of bearing a child into two phases: embryonic and fetal. The first begins from the moment of fertilization and ends at a period of 8 weeks, amounting to a total of 2 months. The following periods are distinguished in the fetus:

Blastogenesis - lasts from the moment of the fusion of the female and male gametes until the 2nd week of pregnancy. At this time, the diploid cell moves through the fallopian tubes to the uterus, after which it is introduced into its mucous membrane.
Embryogenesis - is the period from 2 to 8 weeks of gestation, during which the formation of the placenta and the rudiments of organs occurs.

After two obstetric months, the fetal period begins, which will last until childbirth. Since then, the embryo is considered a fetus. An important feature of this time is organogenesis - the formation and development of all organs and systems of the body.

In the fetal period, the following stages are distinguished:

early (12-28 weeks) - the organs are already formed, now their cells are differentiating, that is, changing their properties;
late (from 28 weeks to the date of birth) - the growth of organ tissues continues, the subcutaneous fat layer of the skin is formed.

The first trimester (up to 13 weeks) accounts for the main critical periods of fetal development.

Obstetric term 2-3 weeks

Most women do not even know about their "interesting" position at this time, and the fetus has the first difficulties on the way to life. A little more than a week has passed since conception, and the diploid cell moves through the fallopian tubes to the uterus. Obstacles may arise on its way: adhesions or insufficient activity of the ciliated epithelium. In this case, it becomes impossible for the embryo to enter the uterine cavity, which forces it to attach itself to the wall of the fallopian tube. Pathological tubal pregnancy develops.

Even if nothing prevents the embryo from advancing, it must enter the uterus at the right time. With a slow path, an ectopic pregnancy will develop, and with an accelerated one, the egg is likely to die due to unavailability for implantation.

Let's say the embryo successfully traveled through the fallopian tubes and reached the uterine cavity at the right time. Now the fertilized egg should attach to the wall of the organ. For this, it is necessary that the uterus has a healthy epithelium, and the embryo itself is strong enough. At this stage, natural selection occurs: if the embryo is too weak or has developmental pathologies, the mother's body rejects it.

What factors adversely affect

Critical periods of fetal development begin almost immediately after fertilization. The newly formed embryo is very sensitive to environmental factors and the state of the mother's body. Experts identify the following reasons that affect the process of implantation of the fetal egg:

individual features of the structure of the uterus;
adhesions as a result of operations on the internal genital organs;
tumors of the uterus and appendages;
severe psycho-emotional state;
physical activity, overwork;
alcohol and drug use, smoking;
taking certain medications;
endometrial diseases.

The risk factor is women with a history of abortion, miscarriage, ectopic pregnancy.

Critical periods require special care from the expectant mother. If the pregnancy was not planned, at this time there is little that can be done without knowing about your situation. Otherwise, a woman should carefully prepare for conception: undergo an examination, cure existing diseases of the genitourinary system, not burden herself with unnecessary work and take care of emotional stability and comfort.

Risks for a period of 4-7 weeks

The embryo successfully attached to the wall of the uterus and continued its development. Of the two, a fetus and a placenta will form. It would seem that the most difficult stage in the life of the embryo is behind, but no. Critical periods in the development of the embryo and fetus reappear at the obstetric period of 4-7 weeks. The future mother already knows about her situation.

There is an active development of the embryo: the rudiments of organs and systems are formed. At this stage, the embryo is very sensitive to the external environment, the risk of developmental pathology is high. A pregnant woman should carefully listen to the advice and warnings of doctors and take care of herself: even a mild cold can call into question the bearing of a child.

Critical period for a period of 8-12 weeks

At this time, the influence of external factors on the development of the embryo is one of the main dangers. Meanwhile, changes in the mother's body are also important, which is actively tuned to bear the fetus. Formed after ovulation, the corpus luteum produces progesterone, which contributes to the development of pregnancy. By week 12, it "transfers" its functions to the placenta. This is a very risky moment, because the amount of the hormone may not be enough for further gestation. By registering with a gynecologist, this is easy to avoid: with the help of laboratory tests, progesterone levels are kept under control.

For the formation of the placenta, it is important to lead a healthy lifestyle, eat a balanced diet. Take any medication only with the permission of a doctor and under his supervision.

Risk factors in the first trimester

The main critical periods of intrauterine development of the fetus by months fall on the first trimester. At the initial stage, both the embryo and the woman in the role of mother are weak. Any external influence calls into question the life of the embryo. In order not to once again put the pregnancy at risk, doctors recommend to beware of:

Taking medication - the contents of the first aid kit will have to be carefully reviewed. For example, only paracetamol is allowed as an antipyretic and analgesic, and you should forget about acetylsalicylic acid, as well as about preparations based on it (analgin, aspirin). As an antispasmodic, use no-shpu (drotaverine). Among antibiotics, only penicillin series are allowed.
Medicinal herbs and traditional medicine - most medicinal fees, teas are prohibited during pregnancy and lactation. Before using such drugs, it is better to consult a doctor.
Stress, psycho-emotional disorders - the state of the mother directly affects the child, which is most dangerous in the first months of pregnancy.
Physical activity - do not exhaust yourself at work or carry heavy loads. The expectant mother needs rest and reasonable physical activity;
drinking alcohol, drugs, smoking - the toxic effects of poisons cause serious pathologies in the development of the embryo.
Infectious diseases - any virus can seriously harm the fetus.

Critical periods in the development of the embryo and fetus require special care from the mother. If you do not neglect the doctor's recommendations and take care of yourself, reasonably limiting everything harmful and dangerous, then the most difficult first months of pregnancy will be successful for both the mother and the child.

Term 18-22 weeks

What are the critical periods of fetal development after the first trimester? The fifth or sixth month of pregnancy (18-22 weeks of gestation) is considered no less responsible. During this period, the uterus is actively growing, accelerating the pace by about 2 times. Diseases of the mother are no longer so dangerous for the child, but still undesirable. The worst thing in this period is to catch infectious STDs (herpes, toxoplasmosis, chlamydia, ureaplasmosis). However, this happens quite rarely, especially if a woman has only one permanent sexual partner.

The period is also risky for women with isthmic-cervical insufficiency (ICN). This is a condition in which the cervix and isthmus cannot cope with the increasing load on them. As a result, premature disclosure occurs, which often leads to spontaneous miscarriages. However, if a woman is under the supervision of a doctor, then the pathology is diagnosed in time. A mini-operation is performed: a suture is placed on the cervix under local anesthesia. This allows you to carry the baby to the full term of pregnancy.

Critical periods of fetal development between 18-22 weeks of gestation require careful medical monitoring. A woman still needs to protect herself from infections, physical exertion and stress. But this time is no longer so dangerous for the baby, and if the expectant mother responsibly treats visits to the gynecologist, then it is easy to survive without consequences for the health of the fetus.

Risks in the eighth month of pregnancy

At a period of 28-32 weeks, the next critical periods of fetal development begin. Obstetrics notes that this time is no less important for the health of the child. The danger is a premature birth process or fading of pregnancy, which usually occurs due to hormonal disorders in the mother's body. Complications such as placental abruption, late toxicosis are also possible. All this can lead to the death of the fetus before delivery or a stop in its development.

If a woman has a history of miscarriage, the monitoring of the preservation of pregnancy should be carried out especially carefully. There is not much time left ahead, so every effort should be made to bring the baby to at least 38 weeks. Hospitalization in an obstetric hospital is often recommended.

Individual critical deadlines

The main critical periods of gestation were discussed above. It is during these periods that most often difficult situations arise that threaten the life and health of the embryo. But each organism is individual, so doctors identify other dangerous periods of pregnancy. These include weeks during which:

a miscarriage occurred or an abortion was performed;
there was a threat of miscarriage in previous pregnancies of a woman;
there was an excessive physical or psycho-emotional stress;
the woman had menstruation after conception.

This includes the period after the expectant mother has suffered the disease.

What you need to know about the "dangerous" terms of pregnancy?

If the doctor has identified a woman as a risk group, during critical periods, bed rest and sexual rest should be adhered to, or even carried out in an obstetric hospital under round-the-clock supervision of specialists. If at least one of the alarming symptoms occurs, you should immediately consult a doctor:

pain in the sacrum, lower abdomen;
vaginal discharge (spotting, bloody, brown);
suspicion of the disappearance of signs of pregnancy;
lack of fetal movements or activity in the last trimester;
excretion of amniotic fluid.

How to successfully survive critical times?

Critical periods of fetal development and their characteristics warn the expectant mother about possible risks and factors of their occurrence. Having learned about pregnancy and deciding to keep it, you should responsibly approach the pace of your life. It is much easier to avoid trouble if a woman regularly visits a doctor, eats fully and rests. All the hard work will have to be entrusted to other family members, unloading your day to the maximum. You will also need to protect yourself from harmful factors (sources of infections, useless and dangerous products, toxic substances).

Critical periods of fetal development are the most dangerous time during which the embryo is most susceptible to environmental influences. At these times, failures in the maternal body also occur, which often lead to miscarriage or fading of pregnancy. Do not brush aside the "dangerous" periods. Fulfilling all the prescriptions of doctors and protecting herself in these difficult weeks, a woman will not only successfully survive them, but will also significantly increase the chances of giving birth to a strong and healthy baby on time.

So, the whole process of formation of the functional system mother - placenta - fetus occurs unevenly. It is possible to single out the periods of greatest sensitivity of developing germ cells (during progenesis) and the embryo (during embryogenesis).

The Austrian physician Norman Gregg (1944) and the Russian embryologist P. G. Svetlov (1960) were the first to note the existence of such periods. The main idea underlying the theses of this theory is that each stage of development of the embryo as a whole and its individual organs begins with a relatively short period of qualitative restructuring. This process of qualitative restructuring is accompanied by the determination, proliferation and differentiation of cells. It is at this time that the greatest damaging effect of many harmful factors (such as X-ray exposure, drugs, etc.) on the development of the fetus is noted.

The following periods are distinguished.

1. In progenesis - spermiogenesis and oogenesis (meiosis).

2. In embryogenesis:

a) fertilization;

b) blastocyst implantation;

c) development of the main organ rudiments and the formation of the placenta (3–8 weeks of development);

d) period of enhanced brain growth (15–20 weeks);

e) formation of the main functional systems of the body and differentiation of the reproductive apparatus (weeks 20–24);

f) fetal period (period of increased growth of the fetus);

g) birth.

3. In the postnatal period:

a) neonatal period (up to one year);

b) the period of puberty (from 7–8 to 17–18 years).

The following unfavorable factors that are unacceptable during critical periods of development should be noted: chemicals (especially industrial and potent drugs), ionizing radiation, hypoxia, insufficient intake of nutrients, alcohol, narcotic substances, viruses, etc.