Maria Sokolova

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As soon as the 2 long-awaited lines appear on the test, and the state of joyful shock passes, the expectant mother begins to calculate the date by which the little one should be born. Of course, knowing exactly the day of conception, it is not difficult to determine the approximate day of birth, but if such data is not available, you can only rely on existing traditional “calculators”. It is clear that the gestational age is almost impossible to calculate down to days and hours (too many factors influence pregnancy), but there are still methods for calculating the most accurate period.

Calculation of obstetric gestational age based on the date of the last menstruation

At a time when there were no high-tech diagnostic methods, doctors used the method of determining the gestational age using “critical days” for such calculations. What is called in medicine “obstetric period”. The method is successfully used today, and involves calculating the period (which is 40 weeks) from the 1st day of the last menstruation.

Obstetricians determine the date of birth in the following ways:

• Date of the 1st day of the last menstruation + 9 months + 7 days.
• Date of the 1st day of the last menstruation + 280 days.

On a note:

This period is approximate. And only one out of 20 mothers will give birth clearly in the week that was calculated by the gynecologist. The remaining 19 will give birth 1-2 weeks later or earlier.

Why can an “obstetric” date be wrong?

• Not every woman has regular “critical days.” The cycle and duration of menstruation are different for each woman. One has 28 days and regularly, without failures, and the other has 29-35 days and “whenever they want.” For one, the torment of menstruation takes only 3 days, while for another it takes a week, or even a week and a half.
• Conception does not always occur exactly at the moment of sexual intercourse. As you know, a sperm is capable of living for several days (or even a week) in the fallopian tube, and on which of these days fertilization occurred - no one will guess or be able to establish.

How to calculate the gestational age based on the first fetal movement?

The oldest, “grandmother’s” method for determining the gestational age. It cannot be considered the most accurate, but together with other methods - why not? The date of the first movement of the baby is still noted in the pregnancy history of the expectant mother.

It's simple: the 1st movement is exactly half the time. For the 1st birth, this usually occurs at week 20 (that is, the date of the 1st movement + another 20 weeks), and for subsequent births - at the 18th week (the date of the 1st movement + another 22 weeks).

However, it should be remembered that...

• The expectant mother will not even feel the first real movements (the baby begins to move already at the 12th week).
• Often, mothers mistake gas formation in the intestines for the first movement.
• A thin, slender mother with a sedentary lifestyle will most likely feel the first points much earlier.

Considering the inadequacy of this method for making important decisions about the timing of birth, relying solely on it is not only naive, but also dangerous. Therefore, determining the date of birth can only be complex. That is, adjusted based on all factors, analyses, diagnostics and other indicators.

We calculate the gestational age and due date based on conception on the days of ovulation

The easiest way to calculate your gestational age is to use the days of ovulation in the calculations. Most likely, pregnancy occurs on the 14th day of a 28-day cycle (or on the 17-18th day in a 35-day cycle) - this day is the starting point for the gestational age. To make calculations, you just need to subtract 13-14 days from the date of the missed period and add 9 months.

The disadvantage of the method is the low accuracy of forecasts:

• 1st reason: the period of sperm activity (2-7 days) in the fallopian tube.
• 2nd reason: it is difficult to determine the approximate day of conception if spouses make love several times a week or more often.

How do obstetricians-gynecologists calculate the gestational age?

On the first visit of the expectant mother with an embarrassed “I’m probably pregnant,” the gynecologist is primarily interested in the date of the last menstruation. But the gestational age will be calculated, of course, not only on its basis, but in a comprehensive manner.

The “package” of such factors and criteria includes the following methods:

According to the size of the uterus

An experienced doctor will very quickly and clearly determine the period using this method, especially in the early stages. For example, during pregnancy up to 4 weeks this criterion will be equal to the size of a chicken egg, and at the 8th week - to the size of a goose egg.

After 12 weeks it is more difficult to determine, because each baby is individual, and the size of the uterus in 2 mothers with the same period may be different.

By ultrasound

Again, before the 12th week of pregnancy, determining its due date is an easier process than starting from the 3rd month.

The error of ultrasound diagnostics from the 2nd trimester is due to the individual development of babies.

According to the height of the uterine fundus (UFH)

The gynecologist uses this method starting from the 2nd trimester of pregnancy. During the process of carrying a baby, the uterus grows with him and gradually extends beyond the bottom of the pelvis.

The doctor measures the UMR by placing the expectant mother on the couch - he probes the uterus through the abdominal cavity and works with a “centimeter” (from the symphysis pubis to the highest point of the uterus). The increase in GMR occurs weekly and most often corresponds to certain indicators.

Deviations of 2-4 cm are possible taking into account the mother’s age, the amount of water and the number of embryos, the size of the baby, etc. Therefore, the obtained indicators must be compared with the size of the fetus and the mother’s waist circumference.

VDM – calculation by week:

• 8-9th week

The uterus within the pelvis. VDM – 8-9 cm.

• 10-13th week

From the 12th week, the development of the placenta, the formation of blood vessels in the fetus, and the growth of the uterus begin. VDM – 10-11 cm.

• 16-17th week

The baby is no longer just a “tadpole”, but a little man with all his organs. UMD - 14-18 cm. At the 16th week, the doctor already palpates the uterus in the area between the navel and pubis.

• 18-19th week

The placental system, limbs, cerebellum, as well as the immune system are formed. VDM – 18-19 cm.

• Week 20

At this period, the VDM should be equal to the term - 20 cm.

• 21st week

From this moment on, 1 cm/week is added. The fundus of the uterus can be felt at a distance of 2 fingers from the navel. VDM - about 21 cm.

• 22-24th week

The fundus of the uterus is narrower in the navel area and can be easily determined by a doctor. The fruit already weighs about 600 g. VDM is 23-24 cm.

• 25-27th week

VDM – 25-28 cm.

• 28-30th week

VDM is 28-31 cm.

• From the 32nd week, the doctor determines the fundus of the uterus between the navel and the xiphoid process of the chest. VDM – 32 cm.
• By the 36th week, the uterine fundus can already be felt on the line that connects the costal arches. VDM is 36-37 cm.
• 39th week. During this period, the fundus of the uterus descends. The baby's weight exceeds 2 kg. VDM is 36-38 cm.
• 40th week. Now the fundus of the uterus can again be felt between the ribs and the navel, and the UMR sometimes decreases to 32 cm. This is the period when the baby is ready to be born.

By head size and fruit length

For this method of calculating the period, various formulas are used:

• Jordania method

Here the formula is presented as X (term in weeks) = L (child’s length, cm) + C (D head, cm).

• Skulsky method

The formula is as follows: X (term in months) = (L x 2) – 5 / 5. In this case, L is the length of the child in cm, the five in the numerator indicates the thickness of the uterine wall, and the five in the denominator is a special coefficient.

The pregnancy calculator allows you to calculate the exact due date and find out what week of pregnancy you are currently in, as well as your expected due date. To calculate, select the date of the first day of your last menstruation at the top of the calculator and click on the "Calculate" button. The calculator will then automatically display your current week of pregnancy and your estimated due date.

For each week of pregnancy, two exact dates are displayed: the start day and the end day. The interval between them is exactly 7 days. You can scroll through the weeks of pregnancy using the arrows. A complete list of weeks from 1 to 40 is at the very bottom of the calculator. By clicking on them, you can also scroll through the slider to the desired week. The trimester of pregnancy to which each week belongs is indicated by Roman numerals at the bottom (at the slider) and in the upper right corner of the cells (in the full list of weeks). Also, trimesters are indicated in pink, orange and blue respectively. In general, the very concept of pregnancy trimesters is quite arbitrary and their boundaries are not clearly defined. We took as a basis the most common classification, in which the second trimester begins at the 14th week and ends at the 28th. Only your obstetrician-gynecologist can calculate trimesters more accurately after an ultrasound examination.

In the images you can see an approximate illustration of the baby's development inside the womb for each week. To the left of the images is a brief description of fetal growth for that week. You can find more complete and detailed information by clicking on the “More details” link, which is located under each description.

Method for calculating gestational age

The calculator calculates the so-called “obstetric” gestational age. It differs from the actual period by about two weeks. This calculation method is used in gynecology, because It is very difficult to calculate the exact day of conception. Therefore, the calculation is based on the first day of the last menstruation, the date of which, as a rule, every woman remembers. 280 days or 40 weeks are added to this day. Thus, during the first two weeks of the obstetric period you are not yet pregnant, because... ovulation has not yet occurred (based on the average duration of the menstrual cycle of 28 days). This method is not completely accurate, since some women ovulate late or early. But, in general, for most expectant mothers such a calculation turns out to be quite accurate.

In any case, we draw your attention to the fact that the pregnancy calculator allows you to calculate the due date using the most general formula, which does not take into account the individual characteristics of your body. It cannot be used for self-diagnosis or any other medical purposes, since the results of its calculations are for informational purposes only. All precise calculations and any other manipulations should be carried out only by your attending physician.

The birth and development of a new life in a woman’s womb is an exciting and responsible period in a woman’s life. It is almost impossible to know exactly the day of conception. How to count down the cherished 40 weeks of waiting for a baby? To calculate and determine the gestational age, obstetrician-gynecologists use the term “obstetric gestational age.” What does this concept hide, what is the calculation of the obstetric gestational age? Does the actual embryonic development period of the baby differ from the gestational (obstetric) period?

Already on the first visit to the gynecologist regarding pregnancy, a woman is faced with the concept of obstetric term.

How is the obstetric gestational age determined?

The term “obstetric period” of pregnancy arose due to the fact that it is almost never possible to accurately determine the date of conception. In this case, it is necessary to monitor the pregnancy - perform tests, conduct examinations. For each diagnostic procedure, certain deadlines are established. In addition, with each day of pregnancy, the toddler’s body grows and develops. The developmental norms of the baby are also determined in accordance with the gestational age. How to calculate it in each case? In order to systematize the timing of pregnancy, the term “obstetric period” was introduced, the beginning of which coincides with the first day of the last menstrual cycle. That is why, when registering for pregnancy, the gynecologist asks to name the exact date of the last menstruation. The obstetric gestation period is used throughout the entire period of gestation of the toddler. All medical documents and formulations refer to the obstetric period.

Obstetric gestational age and real

Despite the fact that the obstetric period is the main one (for doctors) throughout the entire expectation of a toddler, everyone understands that in fact the pregnancy occurred after the first day of menstruation. What is the real age of a developing baby? With the most common cycle length - 28 days - ovulation normally occurs on day 14 (± 2 days). It is known that the fusion of sperm and egg, as well as fertilization of the latter (i.e., successful conception) is possible during ovulation, as well as 2-3 before and 1-2 days after it. If a woman monitored her ovulation, then the moment of successful fertilization can be determined quite accurately. In other cases, the starting point is the duration of the menstrual cycle and the duration of the second phase (normally no more than 16, but not less than 10 days). The date of ovulation is calculated as the difference between the duration of the cycle and the length of the second phase. It is from this date that the real development period of the toddler can be counted. In practice, this calculation is not used, because even in the absence of health problems, ovulation may occur a little earlier or later than the “due date.”

Why is the gestational age according to ultrasound longer than the obstetric one?

In addition to obstetric and real gestational age, there is also such a concept as gestational age according to ultrasound. How is this value calculated?

• In the early stages of expecting a baby, ultrasound diagnostics are carried out using a transvaginal sensor. If the embryo has not yet been visualized, the gestational age is set in accordance with the size of the fertilized egg (its diameter).
• From the moment the embryo appears, the gestational age is calculated based on the CTP data - the coccygeal-parietal size of the baby. This is the greatest distance from the baby's head to his tailbone.
• At later stages (during the first and second screenings from 11 to 14 and 18-21 weeks, respectively), the gestational age is mostly clarified. Data is calculated based on the circumference of the head and abdomen, as well as the length of the femur.

The results are interpreted in obstetric weeks of pregnancy. And if, when determining the gestational age by ultrasound in the first trimester, the data with the obstetric period calculated by monthly most often coincide, then in later weeks the ultrasound data may deviate upward. This situation can occur, for example, if there is a large fruit (after all, it is its size that is taken as the basis for the calculation). That is why the most informative regarding the accuracy of the calculated gestational age is considered to be the data obtained during the first pregnancy screening. Although, with the onset of early ovulation, the period according to ultrasound may be longer than the obstetric one already on ultrasound of the first trimester.

Obstetric and embryonic stages of pregnancy - how these concepts differ

So, the obstetric gestational age determines the period of bearing the baby from the first day of the last menstruation. What does the term “embryonic period” of development mean? This concept directly determines the age of the embryo (later the baby). Accordingly, to accurately determine the embryonic gestational age, it is necessary to know the exact date of conception. If the pregnancy is planned, moreover, the expectant mother monitored ovulation, it is quite possible to determine such a date more or less accurately. In the general case, the embryonic period is calculated from the date of ovulation, and, of course, the real gestational age is always less than the obstetric period. How different these periods are depends on the length of the menstrual cycle. So:

• If the cycle is standard 28 days, the difference is about 14 days.
• If the cycle is 21 days, the difference may be 7-10 days.
• With a longer cycle (35 days), the difference can be as much as 2-3 weeks.

The embryonic gestational age, by and large, can only be of interest to the expectant mother.

Obstetric gestational age calculator

The calculation of the obstetric “age” of pregnancy is made in the number of complete weeks and days. A pregnant woman can independently determine this period. To do this, just count the number of weeks and days from the date of your last menstruation. If you wish, you can also use online resources, for example:

Obstetric pregnancy periods by week and calculation of PDR

Based on obstetric gestational age data, it is also possible to calculate the expected date of birth of the toddler - PDR. According to generally accepted norms, the duration of bearing a baby is 40 obstetric weeks. At the same time, the onset of labor in the period from the 38th to the 42nd week of pregnancy is also considered the norm. Knowing the date of the start of the last menstruation (namely, the obstetric period begins from it), you can calculate the date of birth. You can do this in several ways:

• From the first day of your last period, count forward 280 days.
• Subtract 3 months from the first to the last cycle and add 7 days.
• Add 9 months and 7 days to the first day of your period.

1. Stepping back 3 months, we get February 1st.
2. Add 7 days, we get February 8th.
3. PDR - February 8, 2019

To summarize, it is worth saying that the obstetric gestational age must be calculated based on the start date of the last menstrual cycle. It is this parameter that is used by doctors when assessing the development of the baby and the course of pregnancy, determining the date of the necessary studies and, most importantly, when determining the date of birth of the toddler.

In order to avoid any confusion in calculations and observations, gynecologists “invented” the obstetric gestational age, which greatly facilitates their management of pregnancy. But this is very confusing for pregnant women themselves. Many women (if not almost all) find it difficult to navigate these calculations at first. Therefore, today we will study how the obstetric period is calculated and what it can mean if it differs from the ultrasound results.

How to calculate the obstetric gestation period of a child

In fact, everything is extremely simple and really extremely convenient. When determining the duration of pregnancy, obstetricians count not from the moment of conception (that is, the real age of the fetus), but from the first day of the last menstruation (that is, the period when there was no sign of pregnancy yet). Thus, the obstetric gestational age is the number of complete weeks at the moment, starting from the day when the woman began menstruation in the cycle preceding pregnancy. And it's very easy to calculate.

Almost any woman can remember the date when her last period began. It is from this that the obstetric term is calculated. But the exact date of conception of a child is extremely rarely known. Firstly, a woman cannot always accurately determine which sexual act was fatal. Secondly, even if this date is known, we do not know for sure when ovulation occurred in this cycle (even in a healthy woman it can shift in any direction, and the duration of the cycle is different for everyone), at what point the fusion of the egg with sperm (because this probability persists for several days in each cycle), how many days the fertilized egg moved towards the uterus and when it became embedded in its wall (this can be influenced by a large number of factors).

This is one of the reasons why the gestational age is calculated according to this principle. There are other reasons too. The main one has its roots in the distant past. Our ancestors calculated that from the day on which a pregnant woman's last period began until the birth of the child, an average of 280 days (or 40 weeks) passed. This calculation was transferred to obstetric practice, where pregnancy lasts 10 obstetric months, each of which consists of 4 weeks or 28 days. By adding them to the date of the last menstruation, you can calculate the expected date of birth. Another way to calculate the maximum allowance: from the first day of the last menstruation, count back 3 months and add 7 days.

Also, using this method, doctors determine when to register a woman and send her on maternity leave, prescribe various types of examinations, and how to assess the developmental well-being of the baby’s future.

In addition, it is the obstetric period that the doctor focuses on when conducting an ultrasound and assessing the norms of fetal development. This approach allows us to optimize this method, that is, to make it “convenient” for a comprehensive examination of a woman and management of pregnancy.

Obstetric gestational age for abortion

Another reason why this method of calculation may be convenient and “useful” is determining the timing at which artificial termination of pregnancy is possible. Thus, according to the current legislation of the Russian Federation, abortions, depending on the method of implementation, are allowed to be carried out in the following weeks:

• medical abortion - no later than the 5th week of pregnancy;
• vacuum aspiration - also only up to 5 obstetric weeks;
• surgical abortion (cleaning) - between 5 and 12 weeks.

Obstetric and gestational, gynecological, embryonic gestational age: what is the difference?

From the explanations it becomes clear that the obstetric period and the real age of the fetus will differ from each other, but the gestational age of the fetus corresponds to the obstetric period. Confusion again? Not really.

Let's start in order. Obstetric term and gestational age are the same thing, synonymous words. Gestation translated from the Latin “gestatio” means “carrying”. This term denotes the age of the fetus, counting it from the first day of the last menstruation, so that there is no confusion due to individual characteristics, nuances and differences between different women.

There is no such thing as a gynecological gestational age. But an experienced gynecologist, during an examination of a woman in a gynecological chair, is able to determine the approximate term based on the size of the uterus from the first weeks of pregnancy.

The embryonic period corresponds to the real age of the fetus, that is, it is counted from the moment of conception, when the fertilized egg is formed. Since this is possible only on the days of ovulation, and for most women it occurs on days 13-15 of the menstrual cycle, the embryonic gestation period is usually 2 weeks less than the obstetric period (in some cases this difference can reach 3-4 weeks and even more). This results in a discrepancy between your calculations (if you know the approximate date of conception) and the gynecologist’s calculations or ultrasound data: when conducting an ultrasound examination, the specialist also focuses on obstetric weeks.

What does it mean if the obstetric date does not match the ultrasound data?

You can often hear that the most accurate way to determine the duration of pregnancy is ultrasound. In reality, ultrasound is unable to determine the exact age of the fetus, but it reliably estimates its size. Up to 12 weeks, the diameter of the fetal egg and the coccyx-parietal size (the length of the embryo from the crown to the coccyx) are analyzed, then the parameters of individual parts of the child’s body (head and abdominal circumference, hip length). The ultrasound specialist compares the results obtained with data from tables specially developed for this purpose, in which certain parameters of the embryo or fetus correspond to a certain obstetric period.

It should be borne in mind that in the early stages all babies develop at almost the same pace, but as they grow, they acquire more and more individual traits and characteristics - and the differences in height, weight, and head circumference may become more significant. Consequently, it is believed that the gestational age determined by ultrasound before 12 weeks is the most accurate, and it is this that obstetricians advise to focus on in the future. You just need to clarify with the uzist what period - embryonic or obstetric - he is focusing on when performing an ultrasound, because in the first weeks of pregnancy it is also possible to determine the real embryonic period: in this case, there will be discrepancies between the ultrasound data and the period set by the gynecologist by approximately 2 weeks.

If the obstetric due date and ultrasound do not coincide (that is, the due date set by the doctor is more or less than according to ultrasound) at an early stage of pregnancy, then additional research must be carried out to exclude any disturbances in the development of the baby. If such discrepancies occur in the second or third trimester, then errors in the calculations may be associated with the physiological characteristics of the unborn baby (large fetus, tall or short child, etc.). Ambiguous situations arise with calculations in women with irregular, very long or very short cycles.

Therefore, never draw any conclusions. The doctor leading the pregnancy must judge the well-being of the fetus’ development, based on the results of various examinations.

Especially for - Elena Semenova

Any woman, upon learning that she is pregnant, immediately goes to the most important specialist - a gynecologist. It is this doctor who examines the woman, prescribes tests and monitors the woman’s condition throughout the entire period of pregnancy. In addition, the gynecologist determines how long the pregnancy is. And it turns out that for some reason it is longer than the real one by as much as 2 weeks. Why does such a difference appear when determining the obstetric gestation period of a baby? Today we will try to figure this out.

Every woman who will soon become a mother tries to maintain a balanced diet, significantly changes her lifestyle, giving up bad habits, visits doctors, undergoes the necessary tests, and so on. However, at the same time, most of all women are interested in what the gestational age is.

Why is the obstetric period determined?

Pregnancy occurs when a woman's egg merges with a man's sperm in the fallopian tube. In most cases, this date coincides with or the day after it. Taking this into account, when calculating the gestation period of a baby from the day of ovulation, it is possible to determine the embryonic duration of development of the unborn baby.

Although this method is not suitable for subsequent monitoring of the progress of pregnancy in every case. This is explained by the fact that ovulation does not always occur in the middle of the menstrual cycle.

In women without any health problems, ovulation may begin earlier or later than expected. And that's quite normal. That is why it is impossible to accurately calculate the embryonic period of a baby’s development.

They began to determine the obstetric period quite a long time ago - in those days they did not even know about ovulation and how pregnancy proceeds. Women determined that pregnancy had occurred only by one sign - their menstruation had disappeared. For this reason, they calculated the gestation period of the child, starting from the day of the last menstruation.

Modern specialists must take this period into account, since it is used to assess the state of growth and development of the fetus.

How is obstetric term calculated?

It is well known that a baby is born after nine months. However, not all women know which months should be taken into account. A month is not an incomplete definition of the duration of bearing a baby. In this case, only the calculation made using the obstetric period is suitable. To do this, you need to mark the last day of your period and count forty weeks or 280 days from it.

The onset of delivery is determined by various methods. For example, the first day of menstruation, which was the last, is taken and 280 days are added to it.

However, experts believe that the surest way is this: add 9 months and 7 days to the first day of your period - the resulting figure will be the date of birth. On this day the period will be 40 weeks. However, this is a tentative date, since they can begin either at 38 weeks of gestation or at 42.

Who determines the obstetric term?

All dates necessary during pregnancy are calculated by an obstetrician-gynecologist.

Of course, many women do not understand how the calculation and basis for recording this period of pregnancy are carried out. Do not panic if it does not coincide with the date that you calculated, since the obstetric period is used to monitor the progress of bearing the baby, order the necessary examinations and important tests. Absolutely all expectant mothers are examined within identical periods.

In some cases, to specify the duration of pregnancy, women are sent to. However, this is not correct. During an ultrasound examination, the specialist assesses the stage of development of the child, based on the obstetric period, since this is what they focus on during all nine months. If the ultrasound data does not match, the doctor may prescribe a new examination.

It must be remembered that the first 14 days before the day of conception also play a special role. During this period, the woman’s egg goes through a preparation stage. Negative external or internal factors that can affect a woman’s health can also affect the course of pregnancy.

That is why the 2 weeks preceding the day of conception are also included in the obstetric period.