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What are obstetric weeks of pregnancy and why are they needed. What is obstetric pregnancy

A desired pregnancy is the most exciting and joyful event in the life of a mother-to-be. Seeing two red stripes on the test, or suspecting a sudden pregnancy, surely every woman is eagerly eager to get an appointment with an antenatal clinic and hear her pregnancy period. When visiting a specialist, the expectant mother will be sure to be told what period in weeks has already come, and what tests and studies she will need to undergo in the future. But what if the announced date does not agree with your personal calculations? How to understand a doctor correctly? But we will now analyze this in detail.

First of all, we note that there are several concepts of the timing of pregnancy. These include:

  • Obstetric
  • Fetal (actual)
  • Term for ultrasound diagnostics.

Now let's take a closer look at each of these concepts.

Obstetric.

It is this gestational age in weeks that the obstetrician-gynecologist announces to the expectant mother. According to the same calculated period, he will guide the expectant mother until the end of her pregnancy, make various appointments and conduct various examinations. Obstetricians even have special universal calendars, cheat sheets, by which they very quickly calculate the date of future birth, starting from the set obstetric term.

This gestational age is considered not from the very moment of the actual fertilization of the egg (conception of the child), but from the first day when the last menstruation was, and this is important to remember and understand. But from the first day of the last menstruation to the actual conception, approximately two weeks have already passed (therefore, the medical and actual dates do not usually coincide by two weeks). This happens because somewhere in about two weeks from the beginning of each menstruation, the ovulatory phase is established, that is, just the very moment when conception in a woman is most possible. In general, the time of ovulation falls in the middle of the monthly cycle, for example, if your cycle is 28 days, then ovulation will most likely happen on the 14th day. Therefore, in the obstetric calculation of pregnancy, there is always about two weeks, when, in fact, conception has not yet occurred. The question arises, why then does the doctor determine not the actual period, but his own, the doctor's? And the thing is that obstetricians all over the world are guided by this period due to several factors. A woman, having come to an appointment, cannot always say with certainty when she had an intimacy, and even if she does, it does not at all guarantee that the sperm fertilized the egg on that very day. Further, the duration of the menstrual cycle is always individual for each woman. Someone may have 25 days, someone 33, but for someone it is far from constant. And therefore, in order not to understand each individual case, and not to calculate the date of ovulation, such a universal system was created that is suitable for all women to determine the obstetric term.

Embryonic (actual).

This is exactly the same period that describes the actually already onset pregnancy, the conception that has occurred. Knowing the obstetric term, each expectant mother will be able to easily calculate her actual one herself, simply by reducing the first by about two weeks. For a more accurate calculation, a woman still needs to know how many days her menstrual cycle lasts, and when ovulation occurs. As soon as the expectant mother begins to suspect that the fact of pregnancy has come, she just needs to calculate when the ovulatory period was, that is, somewhere in a couple of weeks from the last menstruation. This time will be the estimated days of the baby's conception.

Pregnancy period by ultrasound.

It should be noted that ultrasound diagnostics is also considered one of the most reliable methods for determining the duration of pregnancy. In most cases, this method is only additional to the previously set deadlines, and is prescribed for the purpose of monitoring the development and growth of the fetus. But it can also become the main way, in such cases as irregular periods with different duration of the menstrual cycle, or repeated pregnancy immediately after the first birth, when the mother, by giving up breastfeeding, restores the ability to mature the egg.

When determining the duration of pregnancy by ultrasound diagnostics, you need to take into account the period (trimester) when the study took place. If it was done at the very beginning, in the first trimester, then the main indicator on which the doctor relies is the size of the ovum and the coccygeal-parietal size. Focusing on the results obtained, the ultrasound doctor analyzes them with average values, and gives an opinion on the duration of pregnancy.

From the next trimesters, conducting such studies, the specialist measures already the circumference of the head, tummy, thigh length, overall fetal growth, and other important indicators, by which he draws a conclusion about the gestational age.

How to calculate the date of future birth yourself

It is not at all difficult to calculate the date of birth of a future baby, especially since there are several universal formulas for this:

  1. Last day of expired periods + 9 months + 7 days. An illustrative example: The date of the last day of menstruation is March 15th. We add 9 months, we get December 15. We add another 7 days, it turns out December 22.
  2. There is another method: The last day of the past period is 3 months + 7 days. The same illustrative example, only we count differently: The date of the last day of menstruation is March 15th. We subtract 3 months, it comes out on December 15th. We add 7 days, and the result is December 22.

Well, as an afterword, I would like to note that no matter what date of birth the doctors or the expectant mother herself have counted, you should always remember that delivery on time is the period between 38 and 42 weeks. After all, every woman has her own individual organism, just like every future baby has her own special development program, which is laid down only by nature, and therefore only the baby knows when it is best for him to be born.

In the life of every woman there comes a moment when she finds out that she is pregnant. And then, before her and before the doctor in the antenatal clinic, the task will be to correctly determine the gestational age. How to calculate it correctly? After all, a lot depends on this.

Counting techniques will be easy to master on your own. You just need to understand such concepts as obstetric and embryonic (from the formation of the embryo) terms, how they differ and how to define them.

Obstetric term

The gynecologist at the very first examination always asks the pregnant woman about the date of the start of the last menstruation. Starting from this day, the obstetric period is counted: everyone knows 40 weeks.

Despite the fact that this method of counting does not take into account the peculiarities of a woman's menstrual cycle, it is generalized and approximate, gynecologists have been trusting her for a long time. The first of the critical days is the beginning of the life of a new egg, which will soon be fertilized. Therefore, the estimated due date (PDD) and the beginning and end of maternity leave will be calculated according to the obstetric period.

From the date of the start of the last menstruation, 280 days are counted until the PDD. 280 days is 10 lunar months, since the moon makes one revolution in 28 days. This period is equal to 9 calendar months, it is this calculation that is most understandable for everyone.

Embryonic (true) term - from conception

The embryonic term is also called true, because it takes into account ovulation. The formula for calculating it is as follows - another 14 days are added to the date of the onset of menstrual bleeding. Two weeks is the middle of a typical 28-day menstrual cycle. However, not all women have a cycle of exactly this duration, so the moment the egg is released may come earlier or later, in the interval from 12 to 18 days from the first day of the cycle. In the correct determination of the moment of ovulation, the difficulties of this calculation consist.

The primary role in fertilization is played by ovulation, the onset of which has already been learned to determine with the help of tests. It lasts only a few seconds, and the matured egg cell will live in the female body for some more time, but not more than two days. Male reproductive cells also live on average from 2 to 7 days. It is in this interval while the egg and sperm cells are alive, conception will occur. However, medicine cannot yet determine this very day.

With deviations of 2-7 days, the time of conception can be determined due to the opposite situations that are possible in the body. For example, at the end of the second day of its life, the egg finally met a sperm, or vice versa, the sperm waited for seven whole days, but still waited for ovulation.

In order to accurately determine ovulation as much as possible, you need to use a special pharmacy test or do an ultrasound scan. This is usually done when planning a pregnancy.

The well-known definition of basal temperature in the mouth, vagina or rectum can also show the time of the onset of ovulation. It is best to fix the changes early in the morning, right after waking up, without even leaving bed. Before the female cell leaves the follicle, the temperature will drop slightly and then rise. This indicates that ovulation has occurred.

Many women can define this day for themselves without any manipulation of their feelings. The so-called dangerous days come when the lower abdomen begins to pull, vaginal discharge thickens and libido increases.

Thus, an approximate calculation of the embryonic period: the date of the first day of menstruation plus 14 days, is also popular. It takes into account the time of the onset of ovulation and, if correctly determined, can more accurately indicate the date of birth of the child.

For various reasons, many women experience disruptions in their menstrual cycle. In these cases, standard counting methods are not suitable; you have to resort to other methods. Here is a list of them:

  • In the examination chair of the gynecologist
  • When you feel the first movements of the child
  • Depending on the size of the uterus

In particularly uncertain cases, the doctor can use all of these methods to establish the PDD.

Examination by a gynecologist

In the first trimester of pregnancy, a qualified technician can feel the term by touch. Assessing the boundaries of the uterine cavity, it can be concluded that pregnancy has begun (4 weeks) if the uterus is the size of a hen's egg. An enlarged uterus, in proportion to a goose egg, indicates 8 weeks.

Ultrasound

Ultrasound examination nowadays gives very accurate data on the gestational age. The device allows you not only to see the ovum, but also to measure its parameters. In the first 12 weeks, only the diameter of the egg is measured and compared with normal values. In subsequent trimesters, when the fetus becomes human-like, the circumference of the head, chest and tummy is measured.

Most accurately, the ultrasound time can be determined in the early stages, since the more the baby grows, the stronger the genetic traits inherited from the parents are expressed in him: he may be larger or smaller than other children at the same stage of development.

Baby's first thrusts

According to medical research, a pregnant woman will feel the movements of her baby for the first time at the 20th week of pregnancy. If she is expecting a baby for the second, third and further times, then the waiting period for the first tremors is reduced to 18 weeks. Based on this, you can also calculate your term.

However, the accuracy of this technique is controversial. In fact, a tiny baby in the womb makes its first movements before 12 weeks, but it is still so small that the mother does not feel it. Although there are exceptional cases.

Very often, young mothers, pregnant for the first time, mistake gas formation in the intestines for the baby's movements. Especially if they have a tendency to flatulence. But still, as soon as you can say with confidence that you felt the first tremors, be sure to inform the doctor in charge of your pregnancy.

The height of the uterus is equal to the week of pregnancy

Another very convenient way to clarify the term is associated with measuring the height of the uterus. It is carried out with a special meter - a pelvimeter. The pregnant woman herself will not be able to use it, so this procedure is performed by the doctor when she is lying on the couch. The pelvimeter measures the distance from the top to the bottom of the uterus.

The resulting value in centimeters indicates the current week of pregnancy, that is, the height of the uterus of 25 centimeters occurs at 25 weeks.

The combination of all of these methods gives an accurate picture of the date of birth.

Every expectant mother dreams of finding out when her baby will be born. To calculate the DA, there are various calculations, tables that she can use on her own.

Formula Negele

This formula can be applied when a woman has been menstruating regularly without interruption:

  • Option 1. By the day the last menstruation begins, 7 days are added, and another three months are taken away from the result.
  • Option 2. First day of your period plus 9 months and a week.

To simplify the calculation, there are even special pregnancy calendars, where the calculations have already been made. It remains only to determine the date of birth of the baby by its day of the last menstruation.

Practice shows that rarely babies are born on the very day that fell in the calculation. But this is not the most important thing, the main thing is that he was born healthy.

How to calculate the gestational age and day of birth (video)

There are 2 approaches to calculating the duration of pregnancy: obstetric and embryonic. Let's try to figure out in what cases each of them is applied and used.

Obstetric gestational age doctors calculate from the first day of the last menstrual period during pregnancy. Fetal gestational age calculated from date of conception (ovulation)... As a rule, the difference between obstetric and embryonic dates is 2 weeks, but depending on the regularity of the cycle, it may differ.

Obstetric gestation is applied in our pregnancy calendar by week. This was done with the reason not to confuse expectant mothers in the future, since almost all analyzes and examinations are carried out precisely according to the obstetric period of pregnancy and.

The duration of obstetric gestation is usually 280 days, or 40 weeks (or 9 months and 7 days)

A more accurate understanding of the difference in obstetric and embryonic terms can be given by the following picture, where the upper scale corresponds to weeks embryonic, and the lower one - to obstetric terms.

How to determine the due date by obstetric term

According to the obstetric term, it is easy to determine the planned date of birth (SDA).

The duration of obstetric gestation is usually 280 days, or 40 weeks (or 9 months and 7 days). Those. to determine the date of birth, it is worth adding 9 months and 7 days to the date of the last menstruation, but there is also a more convenient formula.

The simplest and most accurate formula for determining the date of birth by obstetric term is to subtract 3 months from the date of the start of the last menstruation and add 7 days. According to many mothers, this is the most accurate indicator.

For example: Your last menstrual period before pregnancy was on June 1st. Subtract 3 months and add 10 days. The planned due date is March 11th.

Determining the duration of pregnancy using ultrasound

The relatively accurate timing of your pregnancy can be determined using ultrasound in the first trimester of pregnancy. From the second semester, due to the structural features of the fetus, it is very problematic to determine the exact date.

With ultrasound, the norm is a difference of 2 weeks with the obstetric period, and it can be both up and down. In case of revealing any inconsistencies, you need to check your calculations of the start of pregnancy. If no error is found, then you should consult a doctor for advice.

As you know, in most cases, it is quite difficult for young girls to establish the exact date of conception. That is why in medical practice, when establishing the duration of pregnancy, they always started from the date of the beginning of the last menstruation preceding pregnancy. With this calculation, the so-called "obstetric" gestational age is established, which is slightly larger and differs from the real one.

How is obstetric pregnancy calculated?

Many women who become pregnant for the first time do not know what the obstetric gestational age means and how to determine it. With a normal duration of the menstrual cycle (28 days), conception is possible by about 14 days. Due to the fact that the date of the last menstruation is used when calculating, then usually the obstetric and embryonic (real) periods of pregnancy do not coincide. The break between them is those same 2 weeks, and sometimes even 3.

How to calculate the embryonic (real) gestational age?

In order for a pregnant woman to independently calculate the actual gestational age, it is necessary to know the exact date of conception. If you cannot install it, then modern ones can come to the rescue. The design of such devices is based on electronic sensors that allow you to accurately determine the duration of pregnancy. In this case, the error is small.

The situation is much simpler when a woman remembers the exact date of her last sexual intercourse. In this case, it is only necessary to calculate how many days have passed since that moment. The resulting number of weeks will be the real gestational age.

How to correctly calculate the duration of your pregnancy?

According to statistics, the difference between the real and obstetric period of 2 weeks is observed in only 20% of pregnant women. For another 20%, the gap between these two periods is less than 14 days. For the majority, 45%, the difference between the two terms fluctuates in the interval of 2-3 weeks, and only in 15% of pregnant women it is more than 3 weeks.

If the average duration of a woman's menstrual cycle differs from the standard 28 days, then fertilization does not occur on day 14, but a little earlier or later. Therefore, the embryonic period will differ sharply from that which the gynecologist will establish.

For example, if a woman's cycle lasts 35 days, then conception can occur only at 21 days, and not as usual, at 14. Therefore, the embryonic gestation period at 1 week of delay will be equal to 5 weeks. Moreover, if you count from the last menstruation, then it will already be 6 weeks.

What to do if you cannot determine the deadline on your own?

In the early stages of pregnancy, it is possible to determine the period quite accurately only with the help of an analysis for. With its help, the approximate age of the fetus is established. In this case, the calculation is carried out from the date of the alleged conception. More accurately allows you to set the time of ultrasound. When conducting this study, the sizes of individual parts of the fetus are taken into account, according to which its age is established. Based on the results of the ultrasound scan, it is possible to establish how the obstetric period of pregnancy, and embryonic.

When determining the gestational age, you can also take into account the duration of the cycle. Indeed, with a longer menstrual cycle, conception occurs a little later, therefore, childbirth will occur later.

Thus, knowing the main differences between obstetric and embryonic gestational age, women will share these 2 concepts, and not be surprised that the period set by the gynecologist is longer than her anticipated one, which is calculated according to the date of conception.

Since it is difficult to determine the exact date of conception in most cases, in order to determine the duration of pregnancy in official medicine, it has always been customary to focus on the date of the beginning of the last menstruation. This method of calculating the gestational age is called "obstetric".

"Obstetric" and "embryonic" terms of pregnancy

However, as it might seem, the “obstetric” gestational age has nothing to do with the date of conception. The birth of a new life occurs much later.

With the usual duration of the menstrual cycle (28) days, conception occurs at about 14 days. In this case, the "embryonic" gestational age will be less than the "obstetric" one by an average of 2 weeks.

Thus, at 4 weeks gestation, the embryo will only be 2 weeks old.

Depending on the individual characteristics and the duration of the cycle, the difference between obstetric and embryonic gestational age for each woman can be individual and averages 2-3 weeks.

According to the results of our survey, this difference is:

- about 2 weeks only in 20% of women,
- in 20% of women - less than 2 weeks,
- in 45% of women - from 2 to 3 weeks,
- for the remaining 15% of women - 3 weeks or more.

How to calculate the correct gestational age?

If your average cycle time is different from the "standard", then most likely fertilization can occur earlier or later than the 14th day of the cycle. This means that the actual gestational age may differ from the one that the doctor will calculate by the date of the last menstruation.

For example: if a woman has a typical cycle duration of 35 days, and not 28, then most likely it will be possible for her to conceive only on about the 21st day of the cycle, and not on the 14th. The actual gestational age at the 1st week of the "delay" in this case will correspond to only the 5th week of pregnancy (or the 3rd week from conception), while the 6th week will already go by the date of the start of the last menstruation!

Our calculator will help you to correctly calculate the gestational age and the age of the fetus, taking into account the individual characteristics of the menstrual cycle.

How to determine the gestational age?

In the first weeks of pregnancy, the safest and most accurate way to determine the duration of pregnancy is with the help of an analysis for hCG. Based on the results of this analysis, you can determine the approximate age of the fetus in relation to the date of conception.

Based on the results of an ultrasound scan, the doctor can put both the correspondence to the obstetric gestational age and the true age of the fetus. In each case, it is necessary to clarify what the doctor meant.

Is it true that the longer the cycle, the longer the pregnancy?

With a longer cycle, conception occurs later, which means that the due date should come later relative to the date of the last menstruation. The average duration of pregnancy relative to the date of conception does not depend on the length of the cycle.