What does monozygotic mean. Identical twins and fraternal twins: "eggs" plus zygotes and pregnancy. How can it be and why

13.10.2019

The birth of twins is not too frequent, accounting for only 2% of the total number of newborns. The birth of triplets occurs even less frequently and accounts for 2% of the total number of twins.

But many couples do not leave hope for multiple pregnancies. How are twins and twins made? Can potential parents somehow influence the process of conceiving several children?

Kinds

To understand how twins and triplets are obtained, one should track the features of the process of conceiving several children. Based on these subtleties, different types of born twins receive certain names.

Identical twins (identical)

They are obtained when an egg is fertilized by a single sperm.

7 days after conception, the female cell divides into zygotes. In this case, the resulting embryos have one placenta and fetal bladder, are in the same amniotic fluid. This option is considered the most typical.

Born twins are very similar to each other in appearance, have similar tastes and habits, and can feel a kind of sacred connection with each other all their lives. Such babies are same-sex, have an identical blood type and Rh factor.

It is interesting! The probability of conceiving identical twins is 0.035%.

Twins

The so-called dizygotic fraternal twins are born more often than identical twins (their number is 66-75% of the total number of born twins). There can be up to 50 births of twins per 1000 births.

Such children appear as a result of the fertilization of two different female cells. At the same time, their maturation can occur in one or both ovaries.

The resulting twins can be both same-sex and opposite-sex and similar to each other no more than ordinary brothers and sisters.

According to medical documents, my twins are dichorionic diamniotic, although at first glance you can’t tell)))

It is interesting! It is noted that in African countries the frequency of birth of twins and twins is the highest. The same figure in the US and Europe is average, and in Japan - low.

royal twins

Often, royal twins are the ultimate dream of most parents. Getting both a boy and a girl at once and enjoying their upbringing is regarded by many as real happiness.

The term "royal" for such twins began to be applied from ancient times, because the simultaneous birth of same-sex children in royal families marked state problems in the future.

Children who were born almost simultaneously could equally claim the throne. Diverse twins solved this problem - the son became the heir to the throne, and the sister became an enviable bride, further providing a profitable dynastic marriage.

For your information! In 100% of cases, royal twins are heterosexual.

In fact, such twins are dizygotic, i.e. 2 eggs were involved in fertilization.

Triplets

Often the conception of triplets occurs through IVF, but in some cases it can occur naturally.

Statistics say that once in 200 female cycles, several eggs mature.

In addition, multiple pregnancy can be the result of involuntary division of one egg. Triplets, like twins, can be same-sex or different-sex. Such children resemble each other no more than ordinary brothers and sisters.

How can this be and why?

Of course, people for many centuries have been interested in the question: in what cases do you get twins or triplets? Moreover, for a long time the birth of such children was considered abnormal.

Modern science refutes this assumption and offers several options for explaining multiple pregnancy.

Genetically (if any)

Hereditary predisposition is the main factor in the occurrence of multiple pregnancy. Most often it is attributed to the female line - twins and triplets can appear in the same genus in different generations.

In a natural way (if it was not in the family)

If there are no women in the family who give birth to twins from generation to generation, multiple conception is rather a happy event.

However, it may depend on some factors:

The age of the woman. Studies have shown that the chances of conceiving and giving birth to twins increase significantly in women from 30 to 40 years old. The reason for this is the failure of the reproductive system - not a single egg may appear in one cycle, but in the next several of them will “jump out” at once.
Lactation. Approximately 6 months after the start of breastfeeding, all the same symptoms can be observed - failures of the reproductive female system. This can happen when the maturation of several eggs is noted. The result is the conception of several babies.
Weight. Another scientifically proven fact is that overweight women have a chance to become a mother of wonderful twins or triplets. The reason for this, paradoxically, is overweight, which contributes to the production of excess amounts of estrogen. It is this hormone that is responsible for ovulation, including multiple ovulation.

IVF

In order to obtain the maximum number of eggs during IVF, an ovulation stimulation procedure is performed. As a result, specialists manage to grow several embryos at once.

In the case of replanting one embryo, the chances of a successful conception of a child are reduced - implantation does not always end successfully. That is why women under the age of 35 are implanted with two embryos, and after 35 years - three.

Further events resemble a lottery - none of the implanted embryos can attach to the uterus, or, on the contrary, all the crumbs will “populate” at once.

For your information! The opinion that the IVF procedure always ends with the appearance of twins is nothing more than a myth. The probability of their birth in such cases is 20%, triplets - about 1%.

When ovulation is stimulated

The procedure for stimulating ovulation is most often performed with medications. The result is often the maturation of several follicles and in the future - the appearance of two or three eggs.

In such cases, multiple pregnancy is only a matter of time.

Rebound effect

This term suggests the appearance of an egg as a result of stopping a course of hormones. Oral contraceptives dull the work of the reproductive system of women, as a result of which the ovaries work in turn, replacing each other in different menstrual cycles.

The abolition of contraception acts on the woman's body as a powerful stimulant - previously "dormant" functions come into a state of combat readiness, there is a powerful release of female hormones into the blood.

At this point, the ovaries can start working at the same time, giving out several eggs at once. However, such an effect is very difficult to trace, since it is spontaneous.

Other factors

Additional factors affecting the conception of several children are:

Abnormal uterus. A doubled or completely divided female genital organ makes it possible for two or more fertilized eggs to grow and develop at once.
Climate. Paradoxically, the possibility of multiple conception is affected by ambient temperature, humidity, day length, or even radioactive background. In different parts of the world, a woman's appendages work differently, in some cases producing several eggs at a time.

Questions

Can it come from one embryo?

It would seem that one embryo implies the birth of one child. However, there are cases when the embryo began to involuntarily divide, and the pregnancy turned out to be multiple.

Most often, this phenomenon occurs as a result of IVF, when the embryo implanted in the uterus (most often at the 5-day "age") is divided into 2 or 3 parts.

Is it possible from different fathers?

Each type of multiple pregnancy is given specific names. So, for example, in medical practice there is the concept of "superfecundation" - a process.

The resulting children are no more alike than typical half-siblings.

The most famous case of the conception of twins from different fathers occurred in an American family under the name of Washington. Boys Jordan and Justin were born a difference of 7 minutes and could well be considered typical twins. Nevertheless, noting the fact that the children are not at all similar to each other, the parents decided on a genetic examination.

The results showed that the boys are the sons of different fathers.

The infidelity of their mother led to the appearance of a very rare phenomenon, since twins from different biological fathers can only be born once in a million cases.

Such a pregnancy usually proceeds typically, and the identification of paternity becomes possible only after a special test.

How to influence gender?

It is not easy to conceive and give birth to several children of a certain gender, but it is possible.

Conception time. Male sex cells are equipped with female and male chromosomes. It is on these indicators that the sex of the unborn baby depends. Chromosomes differ in lifespan and degree of activity. Male - more active, but less tenacious (no more than 24 hours). Female chromosomes are less active, but at the same time they are able to maintain vital activity for up to 5 days. Thus, if a couple wants to conceive boys, the process of fertilization should begin directly at the time of ovulation. To conceive girls, you need to do the opposite - to carry out unprotected intercourse a few days before ovulation. In this case, there is a high chance that male cells will die without waiting for “hour X”, and female cells, on the contrary, having reached their goal.
Pose. Given the above characteristics, you can try to plan the sex of the children by taking a certain position during intercourse. For the conception of boys, it is best to choose positions that involve the easiest and fastest way for sperm to the egg (missionary, "spoons"). If twin girls are a priority, you should act the other way around, choosing the “man behind” position.
Diet. The basic requirement for a diet is the transition of both parents to healthy food. To conceive a boy, you should eat low-fat meat soups and greens, bananas and cereals. Girls "will like" seafood, walnuts and fermented milk products.

What and how to do to conceive twins?

Of course, no doctor will give an absolute guarantee of the conception of two or more babies. However, if some recommendations are followed, a multiple pregnancy can happen.

Parents who dream of leaving the hospital with two envelopes in their hands are advised by doctors to adhere to the following simple recommendations:

Eat more dairy products. Studying multiple pregnancies, scientists have shown that women who regularly consume dairy products are more likely to give birth to twins. But in strict vegetarians and vegans, such cases are much less common.
Sexual activity should not be suppressed. All the same scientists found that a sexually active couple is much more likely to conceive several children. It is in tireless lovers that multiple pregnancies often occur in the first few months after unprotected intercourse.
Planning for twins is best when you already have one child. Researchers are sure that after the birth of one or more babies, multiple pregnancy becomes more real.

There are several options for conceiving twins, their main types are identical ( monozygotic) and multi-egg ( dizygotic, trizygotic etc). All options can be briefly described as follows:

Several spermatozoa have fertilized several eggs (two, three, very rarely more). The twins that appear during such a conception are called polyzygotic, or dizygotic and are no more alike than brother and sister. Indeed, they can even be of different sexes. Each of them has its own separate placenta and fetal bladder.
One spermatozoon fertilized one egg, but after a while the resulting embryo was divided into two identical ones. Such twins are called already identical, or monozygotic because they come from the same egg. They are always of the same sex and are like two peas in a pod because they share the same genetic makeup.
Two spermatozoa (rarely more) fertilized two eggs, and then one of the embryos split into two identical ones. In this case, there will be triplets in the uterus: two absolutely identical and one twin that differs from them.

But the classification of twins does not end there. The fact is that a lot in the intrauterine life of babies depends on their placenta and fetal bladder. The most favorable option is when everyone has their own placenta and fetal bladders are separated by a septum, so that each child develops in his own “apartment”. But, unfortunately, this is not always the case.

Types of multiple pregnancy

Depending on how the placentas and fetal bladders of twins are located, multiple pregnancy is divided into three types:

Bichorionic biamniotic - each baby has its own placenta and its own “apartment” in the fetal bladder. This is the most favorable and safe development option, because the twins do not interfere with each other in any way (mortality does not exceed 9%). This is the most typical variant for multi-ovarian twins, and in the case of identical twins, it occurs in 30% of cases if the separation of the embryo in two occurred within two days after fertilization.
Monochorionic biamniotic - the placenta in this form is one for two, but each baby has its own fetal bladder. At the same time, twins are almost always identical (except in exceptionally rare cases), and among them this type occurs in 70% of cases if the separation of the embryos occurs from 3 to 8 days after conception.
Monochorionic monoamniotic - occurs in 1-2% of identical twins when they separate late, from 9 to 12 days after conception. Both brothers or sisters are attached by umbilical cords to one placenta and swim in one fetal bladder.

If the division of the embryo into two begins even later, after 12 days from conception, then the resulting twins do not have time to separate completely and remain attached to each other by any part of the body ( Siamese twins).

Another interesting pattern is also connected with the period of separation of identical twins: the earlier it happened, the less similar the twins are to each other. That is, if there was one placenta for two, then the brothers (sisters) will have more in common, and if there was one fetal bladder, then it will be difficult even for their own mother to distinguish them.

In order to get multi-egg ( dizygotic) twins, you need at least two eggs at the same time.

Maturation of multiple eggs

Usually, in each menstrual cycle, one egg matures in the ovaries of a woman, while either the left or the right ovary works in turn. During ovulation, it comes out and can be fertilized by sperm.

Sometimes, for reasons not yet explained, maturation of not one, but several eggs occurs in one cycle. In this case, each of them can be fertilized by a separate spermatozoon and form two embryos.

In some cases, doctors themselves stimulate the maturation of a large number of eggs (up to 10 or more) with the help of hormonal drugs. It is called superovulation and is used in assisted reproductive technologies such as .

The natural rhythm of egg maturation is also disrupted when consumed. In this case, after their cancellation, the ovaries can change the mode of operation and produce two or more eggs at once in one cycle. This is the reason for the increased likelihood of twins after using hormonal contraception.

"Double" pregnancy

In some rare cases, in early pregnancy, the ovaries fail for unknown reasons, and after the fertilization of one egg, the second matures. In time, they can stand apart from each other for several days or even weeks. If the second egg is also fertilized, then another embryo joins its counterpart in the uterus. Thus, it is theoretically possible to have two twins at the same time from different fathers.

Sometimes due to the "mixing" of genes in the early stages of development, twins can even have different skin colors (mixed twins, the probability of this case is one in a million). In 2008, a couple from Germany gave birth to two boys, Ryan and Leo, at the same time, one of whom is a pure Negro, and the second is white. Who cares - here: Daily Mail / Proud parents show off their million-to-one black and white twins (eng) .

The appearance of twins is about 1.5% of all newborns. Modern medicine still cannot say exactly why this happens. It is known that the hereditary factor plays a role, for example, 31 women out of 300 had multiple pregnancies in relatives. In addition, the likelihood increases after the abolition of hormonal contraceptives, which have been taken for a long time.

Twins can be identical or fraternal. When, at the very first stage of pregnancy, a fertilized egg is divided into two or more parts, each of which is formed as an independent organism, identical twins, or monozygotic twins, are born.

They are always of the same sex, very similar in appearance, have the same set of genes, often suffer from the same diseases at the same time.

Fraternal twins, or twins, as they are popularly called, develop not from one, but from two eggs that have simultaneously matured in the ovary (ovaries), and at the same time each of them is fertilized by different spermatozoa. Such children are different, like ordinary brothers and sisters born at different times. The genetic structure of these children is completely different.

They can have different blood types and gender, facial features and character. The main difference between fraternal and identical twins is that their genetic similarity is 40-60%, that is, like ordinary brothers and sisters.

Cases of so-called Siamese twins are also widely known. For the first time this phenomenon was stated in Siam (Thailand) at the end of the 19th century. Siamese twins develop from one egg that began dividing at a later date. As a result, the division is incomplete.

Causes of multiple pregnancy

In addition to hereditary predisposition and hormonal drugs, there are several other factors that increase the chances of having multiple children:

Identical twins can be born due to increased production of a hormone that stimulates the maturation and development of eggs;
In women from 35 to 39 years old, the chances of having twins also increase. This is explained by the fact that a kind of hormonal surge occurs before menopause, so late pregnancies are quite often multiple;
Not to mention IVF - in vitro fertilization. According to statistics, often women after IVF have several children at a time. This phenomenon is due to the use of drugs for the treatment of infertility.

Diagnosis of multiple pregnancy

This fact can be established already in the first trimester: the size of the uterus significantly exceeds the norm with a singleton. By the end of the third month, this becomes even more noticeable. You can confirm the assumption with the help of ultrasound. It also happens that for the first time two fetal eggs were found, but at the next planned study, only one. This happens when one of them dies.

With the help of ultrasound, specialists should establish the identity of the fetus - identical or fraternal twins, as well as chorionicity (the number of placentas). The study should be carried out up to a maximum of 14 weeks, since on the basis of the data obtained, the observing doctor draws up a pregnancy management plan.

Also, timely detection of complications characteristic of monochorionic twins depends on this information. For example, feto-fetal transfusion syndrome may occur. In addition, you will need to choose the option of delivery.

The results of the trinity test are not taken into account. The development and condition of the fetus is judged by the indicators of alpha-fetoprotein, chorionic gonadotropin and placental lactogen. These data allow to identify defects and chromosomal pathologies. But with multiple pregnancy, regardless of the number of placentas, much more of these substances are produced.

Multiple pregnancy: features of the course

This phenomenon requires increased attention, since the woman's body is designed to carry and feed one baby. Therefore, twins during pregnancy fall into the so-called biological risk group. In the first trimester, there is a high probability of the death of one of the fetal eggs.

Due to an increase in erythropoiesis in the second half of pregnancy, depletion of iron stores is possible, which provokes anemia. To monitor the condition of the woman and the fetus, a blood smear test is performed.

In a woman, the diaphragm is significantly displaced, the work of the heart and lungs is difficult, there is increased fatigue and shortness of breath, frequent urge to urinate and constipation. Toxicosis, varicose veins also often occur.

In this case, the threat of premature birth or miscarriage in the second and third trimester may develop.

The main reason is excessive stretching of the muscles of the uterus due to the large volume, which provokes labor. Therefore, often women with twins are prescribed drugs to relax the uterus.

To reduce the risk of preterm birth, control the condition of the cervix. If it is shortened, stitches are applied or a pessary is installed.

The bearing of twins is often accompanied by an incorrect position and presentation of the fetus, for example, pelvic, transverse. This factor is decisive in choosing the method of delivery.

To monitor the condition and development of the fetuses, dynamic fetometry and doplerometry are used. It is worth noting that twins can grow differently, for example, one may lag behind in development.

Also, women with multiple pregnancies are more likely to suffer from urinary tract infections. This is due to a violation of the normal outflow of urine and the position of the internal organs due to a significantly enlarged uterus. Such expectant mothers need more careful clinical and laboratory monitoring.

Attention is not bypassed and the weight of the pregnant woman. The difference is that with a singleton pregnancy, the norm is 9-15 kg, but in the case of twins, body weight can increase by another 20 kg. A woman's diet should be balanced, but not high in calories. In the event of edema, the amount of fluid per day is reduced to 1.5 liters.

childbirth process

A woman with twins goes to the maternity hospital a few weeks before the expected date of birth. As a rule, childbirth occurs at 36-37 weeks. The most favorable period for twins is 36-38 weeks, for triplets - 34-36.

The birth process itself can occur naturally. Not always with multiple pregnancies, a caesarean section is prescribed. However, some complications may occur in the process: untimely discharge of water, prolapse of part of the umbilical cord, arms / legs of the fetus.

Sometimes there are weak contractions and attempts due to overstretching of the uterus, fetal hypoxia, postpartum hemorrhage, premature detachment of the placenta of the second placenta after the passage of the first child through the birth canal.

But twin children have a higher adaptability. Their lungs develop and mature earlier than others, which allows them to breathe independently even at early birth. At the same time, identical twins differ from fraternal twins in the early postpartum period: they adapt worse to changes in living conditions.

In addition, they are more likely to develop pathologies associated with respiratory distress, and they are also more prone to icteric and edematous syndrome. Enjoy your pregnancy, easy delivery and health to your twins!

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Pregnancy and the birth of a child is always a miracle, an extraordinary gift. When are two expected? And three? No wonder the twins and their parents so attract the attention of others - sometimes enthusiastic, sometimes sympathetic. This is indeed a rare occurrence. Twins make up about 2% of all newborns, and triplets only 2% of all twins. But before you become a happy owner of a double stroller, you have to go a long way to bear a special, "non-standard" pregnancy.

Children born as a result of multiple pregnancy are called twins. In addition, you can often hear the word "twins", which is colloquially called "dissimilar" twins.

How does it work?

Twins are of two types:
– Dizygotic (two-egg, non-identical) twins develop from two fertilized eggs. In some cases, the simultaneous maturation of two or more eggs occurs in one or both ovaries. A third way of the origin of twins is possible: the fertilization of two or more eggs that have matured in one follicle. Interestingly, sometimes the difference between the time of conception of dizygotic twins can be from several hours to several days. Fraternal twins are no more genetically similar than normal siblings; they are of the same sex and different sexes with about the same frequency.
– Monozygotic (identical, identical) Twins are produced when a single egg is fertilized by a single sperm. Then the resulting zygote begins to divide as usual, but with an interesting feature: as a result of division, independent embryos are subsequently formed. The reasons for the development of this process by science have not yet been precisely established due to the complexity of the biochemical mechanisms that regulate cell division. Such twins are almost exact copies of each other, although, of course, there are still minor differences between them. Indeed, in the initially identical DNA of each embryo, different mutations can occur. The second reason for the differences is non-genetic influences, such as features of the areas of the female uterus or the influence of other factors. Monozygotic twins are 5 times less common than dizygotic twins. Boys predominate among monozygotic twins.

Separate topic - Siamese Twins. So called identical twins, to varying degrees fused with each other. The supposed reason is the late splitting of the embryo into independent forms. The later this happens, the higher the risk. Siamese twins are extremely rare. They are born once in 10 million births.

Many twin conceptions do not end in birth. For every pair of twins born, 10-12 people have a twin in the womb.

Is it possible to plan the birth of twins?

Naturally, almost none. If pregnancy has already begun and the woman knows about it, then in any case it is impossible to change the course of events by any means, because in the process of fertilization, information about the unborn child is laid, including whether he will be alone or there will be two of them.
But some factors still increase the likelihood of conceiving twins, while the likelihood of having identical twins is not associated with any hereditary and external factors and is constant - an average of 3 per 1000 pregnancies, that is, 0.3%.
So, in women aged 35-39 years, the likelihood of such a conception increases. The fact is that in the late childbearing period in the female body, the concentration of the hormone increases, which stimulates the maturation of the egg. Thus, at the same time, several eggs ready for fertilization can immediately mature. It is also known that the duration of daylight hours affects the production of this hormone. Therefore, it is more likely to conceive twins in the spring, when the sun begins to noticeably warm.

If the family already had twins, then the probability of giving birth to twins is higher, the closer the generation in which this happened. The tendency to give birth to fraternal twins is manifested only in women. Men from such families can pass this property on to their daughters, although in the offspring of these men themselves, an increased frequency of twin births is not observed.
More often, several eggs mature in women who have a short menstrual cycle - 20-21 days, as well as in women with anomalies in the development of the uterus, for example, if there is a septum in the uterine cavity or a bicornuate uterus (a pathology of the development of the uterus, in which the organ is not pear-shaped, but as if forked).

In recent years, for the treatment of infertility and other gynecological diseases, hormonal drugs that stimulate the maturation of the egg and ovulation have become widely used. As a result, a long-awaited pregnancy after many years of infertility often turns out to be twins or triplets! Successfully developing in vitro fertilization (IVF) technologies have also led to the fact that pregnancy with twins or triplets is no longer a rarity. Twins after IVF are always fraternal, often of different sexes, each with their own appearance and character.
The frequency of birth of twins is significantly higher during various social upheavals, wars.

When can you find out about twins?

Fifteen or twenty years ago, twins were known only at childbirth or not earlier than the 20th week of pregnancy. When recognizing a multiple pregnancy, the following signs were taken into account.
The increase in the uterus during multiple pregnancy occurs faster than during pregnancy with one fetus, so the size of the uterus does not correspond to the gestational age. At the same time, a significant volume of the pregnant uterus does not correspond to the small size of the presenting fetal head. In addition, with twins, a deepening of the uterine fundus (saddle uterus) can be determined, the formation of which is associated with the protrusion of the corners of the uterus with large parts of the fetus; as well as a longitudinal depression on the anterior wall of the uterus, if the twins are in a longitudinal position, or the presence of a horizontal groove on the anterior wall of the uterus when the twins are in a transverse position. For a long time, it was important for the diagnosis of twins to clearly identify in the uterus during obstetric examination three or more large parts of the fetus (two heads and one pelvic end or two pelvic ends and one head). Equally important was the presence of two points of distinct heartbeat in different places of the uterus.

Cases when not one, but two or more babies settle in the mother's stomach are not so common - 1.5-2.5% of all pregnancies. How does it work? How is a multiple pregnancy different? Can twins be scheduled? Our expert - Anna Romanovna PASTUKHOVA, an obstetrician-gynecologist at the Toast of Family Medicine Centers, answers all questions.

What is the chance of getting pregnant with twins

First, some statistics. As we have already mentioned, this amazing phenomenon occurs in 1.5-2.5% of pregnancies in Europe. It is believed that the appearance of twins, triplets and twins is genetically determined - more often they are born in families where one or both parents were born as a result of multiple pregnancy. However, there are other reasons, including .

The birth of twins occurs with a frequency of geometric progression, obstetricians even have a special formula: for 87 births there is one twin, for 87 twins - one triplet, for 87 triplets - one quadruple, etc. True, today this formula needs to be corrected - multiple pregnancies have become more common.

Identical or identical twins are less common, with an average of 1 in 250 births.

Most often, multiple pregnancies are observed in Africa, least often in Asia.

Types of multiple pregnancy

Let's start with a well-known fact. There are two types of twins: identical or monozygotic and fraternal (they are also called dizygotic).

Monozygotic twins result from the division of one egg fertilized by one sperm. It is they who surprise everyone with their striking resemblance.

Fraternal twins result from the fertilization of several eggs by several sperm. They can be of the same or different sexes, often differ in character and are not always similar in appearance.

Expert comment

A multiple pregnancy is a pregnancy when more than one fetus develops in the mother's body, but several at once. In medicine, identical or fraternal twins (triplets and so on) are distinguished. Identical twins are always the same sex, have the same blood type, and are very similar in appearance. Fraternal twins have phenotypic differences (eye color, facial features), can be either the same or different blood type. Fraternal twins may be of different sexes.

How are identical twins made?

To be honest, we have no idea! The reasons why instead of a single baby from one egg and one sperm there are two (three, four) are not really known - a miracle, and nothing more. The mechanism of the appearance of monozygotic twins, however, is described in sufficient detail. Everything starts as if nothing had happened: one egg, one sperm, meeting, fertilization... Then the resulting zygote begins to divide, grow and, at some point, under the influence of factors that are not entirely clear, it suddenly splits into two or more parts, each of which takes on a life of its own.

Expert comment

The development of identical twins occurs when one egg is fertilized by one sperm, in the process of dividing the fetal egg into two, three or four parts at different stages of its development. In this case, each part develops as a separate organism. It is assumed that such separation can occur when the implantation (attachment) of the fetal egg is delayed due to oxygen deficiency. Also, the occurrence of identical twins is associated with the fertilization of an egg that had two or more nuclei.

Do the twins interfere with each other?

It is very important at what point the splitting of the egg occurs, since the further intrauterine development of the twins depends on this.

There are 3 options for the development of monozygotic twins

Separation of the fetal egg occurred in the first five days after fertilization. In this case, both future babies begin to develop completely independently, each "grows" its own placenta and . Until recently, it was believed that such a development was characteristic only of fraternal twins and identical twins in such cases were erroneously identified as dizygotic (maybe this happened to the Olsen sisters?)

The zygote split approximately 5-8 days after fertilization. By this time, the outer fetal membrane has already begun to form: remains common in babies, however, the inner fetal membrane (amnion) develops in each of them. If babies have a common placenta, then one child can suppress the other - their competition begins with conception.

Separation occurred on day 8 and later (1% of all monozygotic twins). At this time, the fetal bladder is already beginning to form, so the separated embryos will grow in the common amnion and common fetal waters. This situation is considered not the most favorable, there is a risk that the kids each other.

The life of fraternal twins in the womb is organized with great comfort: each of them develops their own personal placenta (sometimes they grow together with walls), which means that babies do not need to seriously compete for a place “under the sun”.

So, if the embryos have different placentas, then most likely they are fraternal, and if the babies “settled” with a common placenta, then these are identical twins who will be born similar as ... twins.

Expert comment

There are the following types of multiple pregnancy:

Monochorionic monoamniotic twins when there is a common placenta and there is no septum between the twins. It's always identical twins

Monochorionic diamniotic twins, when there is a common placenta and septum, i.e. each fetus has its own amniotic sac

Dichorionic diamniotic twins, when each fetus has its own placenta and its own amniotic bladder.

How are fraternal twins made?

Non-identical twins develop from two or more fertilized eggs. How does this happen? The most common reason for this is the so-called "multiple ovulations", when several mature eggs come out of one or both ovaries at once. The reasons for this phenomenon may be different: genetic predisposition, artificial ovarian stimulation, the first cycle after the abolition of hormonal contraceptives.

Do you know that....

The difference in the time of conception of fraternal twins can be from several hours to several days.

In very rare cases, a new pregnancy is added to an existing pregnancy. As a rule, after fertilization and implantation of the fetal egg, ovulation is immediately blocked. If, due to hormonal imbalance, this mechanism does not work immediately, a younger brother or sister is added to the older child in utero.