What is the difference between obstetric period? weekly schedule in the pregnancy calendar. Determination of gestational age

Our calendar is based on the so-called gestational period or obstetric period (that is, it begins counting not from the moment of conception, but from the beginning of the menstrual cycle, which culminated in pregnancy). Therefore, you will actually become pregnant only in the 3rd week of pregnancy.

First week pregnancy. How does the mother feel and what does the baby look like?

Of course, this is not pregnancy yet, since you are having another menstrual bleeding. However, whether you like it or not, the body has begun to prepare for possible motherhood again. Already now your central nervous system actively rearranges hormonal levels. The goal of the rebuild is to gradually prepare one of your 300,000 eggs for fertilization. This takes on average about two weeks. That's why we think this week first: after all, the prototype of your future child is being formed right now!

For now, your baby is more of an idea than a real organism. Its prototype (more precisely, half of the prototype) is one of many thousands of your eggs located in their “cradle” - the ovaries. The second half of the prototype (paternal) has not even had time to form into a mature sperm (this will happen in about two weeks).

Now is the time to understand the timing of pregnancy: obstetric and embryonic, as well as find out the expected date of birth. Let's take a look at these interesting and important issues hurry up!

When is your baby due?

It may seem strange to you to determine the time of birth of a child even before the start of pregnancy, but this is exactly what doctors do. The beginning of pregnancy is usually counted from the first day of the last menstrual cycle. This means that your doctor will determine the moment of conception at least two weeks earlier than it might actually occur. To avoid confusion, let's try to understand this issue.

Obstetric and embryonic stages of pregnancy

In most cases, women do not know the exact date of conception, but they can say exactly when the last one started menstrual cycle. This is the point from which obstetricians usually count pregnancy. .

Pregnancy begins from the moment when the egg meets the sperm in the fallopian tube. Usually this happens on the day of ovulation(in other words, two weeks before the start of the next menstrual cycle) or the next day. Therefore, when calculating the duration of pregnancy from the day of ovulation, we get embryonic period fetal development. But this method is not entirely suitable for further monitoring the course of pregnancy. This is due to the fact that women may not always ovulate in the middle of the cycle. Even healthy women it may happen earlier or later due date. Therefore, it is not possible to accurately predict the embryonic period.

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

Obstetric gestational age

Obstetric gestational age doctors calculate from the first day of the last menstruation during pregnancy.
It is also called the gestational (menstrual, or calendar) age of the fetus. It is on this calendar that doctors and nurses will monitor the development of the fetus. Gestational age is determined by gynecology and ultrasound, counted from the first day of the last menstruation.

Embryonic gestation period calculated from dates of conception (ovulation). It is also called ovulation age (fertilization age, either embryonic or true) is two weeks less and is counted from the actual date of conception.

The obstetric gestational age is applied in our pregnancy calendar by week. This was done for the reason that future mothers would not be confused in the future, since Almost all tests and examinations are carried out precisely according to the obstetric period of pregnancy And.

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

A more accurate understanding of the difference in obstetric and embryonic periods can be given by the following figure, where the upper scale corresponds to embryonic weeks, and the lower scale corresponds to obstetric periods

Many people calculate pregnancy time in weeks. This is the simplest and convenient way to avoid confusion. For example, if your doctor says you are 10 weeks pregnant (remember, counting from the start of your last menstrual cycle), then you conceived approximately 8 weeks ago and will begin labor at 30 weeks because total term pregnancy - 40 weeks.

There is also a large unit of measurement -trimester.
Trimesters divide pregnancy into three phases. Each such phase, lasting approximately 13 weeks, has its own characteristics. For example, miscarriages (spontaneous birth) usually occur in first trimester. This is a period of rapid formation and development of the child’s organs. And in third trimester most likely you have pregnancy-related hypertension (increased blood pressure), toxemia (presence of toxins in the blood) and preeclampsia ( late toxicosis pregnant women).

You may have also heard about another unit of time - lunar month. It corresponds to the shift cycle lunar phases and is 28 days. Full term Pregnancy equal to 280 days is 10 lunar months.

Calculation of due date. How is the expected date of birth (EDD) determined?

Based on the obstetric period, it is easy to determine the planned date of birth.

The duration of obstetric pregnancy is usually 280 days, or 40 weeks (or 9 months and 7 days). That is, to determine the date of birth, you should add 9 months and 7 days to the date of your last menstruation.

Definition of PDR obstetric method
The most famous (and often used) method of calculating the PDR is the Naegele formula or calculation based on the date of the last menstruation. Using this formula, special obstetric calendars, and the period itself is called “obstetric”.
Naegele's formula: subtract 3 months from the first day of the last menstruation and add 7 days. That is, exactly 40 weeks are added to the first day.
For example, the first day of the last menstruation is August 1, we add 7 days to this date and, getting August 8, we subtract 3 months: PDR - May 8.
This option is the most common, but far from the most accurate. Moreover, many women know the date conception and counting the period from it, they are surprised to discover that their PPD often does not coincide with the obstetric one.

40 week chart in the pregnancy calendar

Obstetrician Glade Curtis based the 40-week calendar on a 40-week pregnancy chart.

As mentioned above, according to this schedule actual conception occurs in the third week.

Therefore, all the details of pregnancy are considered from week to week, starting from the third week. Expected time of delivery ( OVR or PDR(estimated date of birth)) falls at the end of the 40th week.

Each week contains two fetal ages.

For example, in the article “Week 8” you will see:

8th week of pregnancy (gestational age) and fetal age - 6 weeks(fertility age).

This way you will know the age developing fetus at any time during pregnancy.

It is important to understand that OVR is an approximate, not an exact date. Only one in 20 women gives birth on the exact day of their due date, and 90 percent of women give birth a week earlier or a week later. Therefore, you cannot rely on the OVR date. It may turn out that it will come, and the child will not be born for some time. Consider this date as a guideline - a deadline for which you must prepare, systematically approaching it.

There is no fundamental difference in how to count the time of pregnancy. All the same, the process will not go faster. It will last as long as nature allows. After all, a miracle happens - it grows and develops in your body new life!

How long does each trimester of pregnancy last?

Officially, trimesters have no clear boundaries. It is generally accepted that each of them lasts approximately three months.

Does pregnancy really last nine months?

On average, pregnancy lasts nine months, but this is not always the case. For example, if the first day of the last menstrual period fell on January 1, the MDA will be October 8, or October 7 if the year is a leap year. It turns out that the pregnancy will last nine months and one week (or even longer if childbirth occurs after the PDR).

How to determine the month of pregnancy?

Look at the table above to see what week you are pregnant. This way you can approximately determine the month of pregnancy. Keep in mind that the first month of pregnancy occurs after one month has passed since the first to last menstrual period (LMP).

Doesn't a month consist of four weeks?

In fact, the only month that has four weeks (or 28 days) is February (unless the year is a leap year). Other months have 30 or 31 days. It turns out that on average each month consists of 30.4 days or 4.3 weeks.

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How to calculate gestational age by week

What is gestational age and how is it calculated? It is customary to distinguish between obstetric and embryonic periods, and many mothers are simply confused about these terms, and today we will try to figure it out.

The first thing we'll start with is obstetric term, because in fact, our gynecologist will make all the calculations based on the obstetric period.

Obstetric term
The obstetric period is calculated from the first day of the last menstruation, so when you go to the gynecologist for the first time, clearly know the date of your last menstruation, this will make your life easier, and in the future all this data will be entered into a special card, everything will be written there.
The obstetric period assumes 280 days of pregnancy, well, these are average figures, it is clear that each mother is an individual and this period may vary, but it is generally accepted to be 280 days.
According to the obstetric period, they issue directions for tests, a certificate of maternity leave and all other documents that you will need in the future, tighter exchange card, there all the data will be based on the obstetric period.
And based on the obstetric period, the expected date of birth is calculated - this is the EDA.

How to calculate your expected due date

Embryonic term
It is calculated from the moment of conception and is usually shorter by 14 - 15 days of the obstetric period.
That is, in fact, we all know biology, menstruation has passed and a new cycle of egg formation has begun, and on the 14th-15th day on average it matures and is ready for fertilization, this is called the day of ovulation.
In fact, it will be very difficult for us to determine and calculate the exact day of ovulation, there is of course different tests at the pharmacy, you can buy it and know exactly when you are ovulating,
but in principle it is very difficult to determine or calculate, because fertilization itself occurs on the day of ovulation, plus or minus 3-5 days.

A woman's gestational age is on average 38 weeks from the moment of conception. For most women, pregnancy lasts 266 days. But it is impossible to accurately calculate the date of future birth until the day. Much depends on hormonal levels women, concomitant diseases mother and fetus, gender and weight of the unborn child, etc. But after the 37th week of pregnancy, the fetus is ready for independent life(full term). After this period, the born child is fully viable.

But a child after 42 weeks of pregnancy is considered, and childbirth can be accompanied by serious complications for the fetus. Therefore, it is important to know the gestational age not so much for the expected date of birth, but rather in order to know at what period the birth will be considered normal for a woman, and the child will be considered full-term.

Obstetric and embryonic gestational age - differences

The obstetric gestation period is 40 weeks, and the embryonic gestation period is only 38. This makes a difference of 12-14 days. The obstetric period of pregnancy begins on the first day of the last menstruation. The embryonic period begins from the day of conception (from the day of ovulation, which usually occurs on the 14th day from the start of menstruation, plus or minus 4 days).

How to calculate the obstetric gestational age?

The obstetric gestational age and the real (embryonic) gestational age differ by 2 weeks. In practice, the embryonic period is not counted and is limited to calculating only the obstetric period. If a woman knows not only the date of the start of her last period, but also the date of conception, then the embryonic pregnancy line is more accurate. The obstetric period of pregnancy lasts 280 days from the first day of the last menstruation. According to the results of ultrasound, according to the tables, they confirm that the fetus corresponds to the obstetric, but not the embryonic, stage of pregnancy.

Is it possible to calculate the date of birth using the obstetric pregnancy line?

The most in a simple way To calculate the expected date of birth, the following can be considered: from the first day of the last menstruation, add 280 days (Keller formula). However, in practice this is difficult and the possible date of birth is determined by two similar methods.

1. Nine months and seven days are added to the start date of the last menstruation.
2. Three months are subtracted from the date of the last menstruation and seven days are added.

Weeks according to the obstetric period of pregnancy begin to be counted from the first day of the last menstruation. For the convenience of the doctor, 40 weeks are further divided into 3 trimesters. The 1st trimester includes 1-14 weeks of pregnancy, the 2nd trimester - 16-28 weeks, and the 3rd trimester - from 29 to 40.

Obstetric gestational age and ultrasound gestational age

It is wrong to think that ultrasound determines the obstetric or embryonic stage of pregnancy. Rather, on the contrary, according to special tables in which the average size of the fetus is entered according to obstetric weeks pregnancy, determine their compliance with the obstetric gestational age. Often the size of the fetus corresponds to the obstetric period plus or minus one week: the fetus develops normally. If the ultrasound term is less than the obstetric term, this does not mean that the obstetric period was calculated incorrectly; rather, something is interfering normal development fetus Main reasons intrauterine retention fetal development:

If the term according to ultrasound is longer than the obstetric one, then most often the reason will be the high weight of the unborn child (due to heredity, diabetes mellitus, maternal overeating during pregnancy).

It is possible that the date of the last menstruation was determined incorrectly by the woman and, if she remembers the date, then it is better to recalculate the obstetric period through the embryonic period, adding two weeks to the latter.

While carrying her first baby, a woman faces many questions. Most of them are related to how she should feel and how she should react to certain changes occurring in her body. But in addition, many expectant mothers are faced with the problem of determining the exact duration of pregnancy. And the numbers given in the results of ultrasound and gynecological examination may differ. Usually, when making calculations, doctors use the obstetric gestational age, but in some cases the real term may also be used. Let’s try to figure out which calculation is right for you? And how to correctly calculate the gestational age?

In most cases, determine the exact date conception is not possible, which is why, to understand the duration of pregnancy, doctors have always relied on the date when pregnancy began. last menstruation. This method of calculating the gestational age is called obstetric. However, such calculations have nothing to do with the date of conception, because a new life is born much later.

As you know, pregnancy occurs at the moment when the egg fuses with the sperm inside fallopian tube. In most cases, such an event occurs exactly on the day of ovulation or within 24 hours after it occurs. Thus, by calculating the duration of pregnancy from the moment of ovulation, it is possible to more or less accurately determine the real period of fetal development.

However this technique is not always suitable for further monitoring of the course of pregnancy, since in many women ovulation does not always occur exactly in the middle of the cycle. In absolutely healthy representatives of the fair sex, this event can happen a little ahead of schedule or vice versa a little later. Therefore, the real (embryonic) gestational age is often simply impossible to determine.

They began to calculate the obstetric period a long time ago, when there was no information about the characteristics of the course of pregnancy, as well as about ovulation. Many years ago, the only symptom of impending conception for a woman was the absence of menstruation, and therefore the period was counted from the moment of her last menstruation.

Obstetric gestational age and real calculation - the difference

The standard menstrual cycle lasts for four weeks to twenty-eight days. In this case, conception occurs somewhere on the fourteenth day, just at this time ovulation is maximum possible. Accordingly, the actual period of fetal development and the obstetric one will differ by about two weeks.

Accordingly, if the doctor tells you that your pregnancy is four weeks, then real age the embryo is only two weeks old.

However, depending on individual characteristics such a difference can be greater or less.

If the length of your menstrual cycle is slightly different from the standard, most likely fertilization occurred a little earlier or later than the fourteenth day of the cycle. Accordingly, the actual gestational age will be slightly different than when calculated by the date of the last menstruation.

For example, if average duration cycle is not twenty-eight, but thirty-five days, then conception can occur only on the twenty-first day of the cycle, and not on the fourteenth. Accordingly, in the first week of delay, the actual period will be the third week, and according to the date of the last menstruation it will already be the sixth week.

When trying to correctly calculate the gestational age, you should not focus solely on the date of sexual intercourse. As practice shows, sperm can remain active in the female genital tract for up to five days, waiting for the egg to mature and fertilization to occur.

How else can you determine the deadline?

In the very first weeks after conception, for maximum precise definition during pregnancy you can get tested for hCG. The results obtained will make it possible to determine the most accurate age of the fetus.

To determine the period at a later stage of pregnancy, you can sign up for an ultrasound. Until the twelfth week of gestation, such a study extremely accurately shows the age of the baby, since its development occurs approximately the same in all women. In this case, the doctor can write as a result both the true (real) age of the fetus and the obstetric gestational age. Accordingly, it is necessary to clarify with the specialist conducting the research what exactly he means.

How is the gestational age determined during a routine gynecological examination?

A qualified gynecologist, conducting a vaginal examination of a patient, can detect pregnancy at the most early stages development, from about the third or fourth week. But of course, the dates he named cannot be called 100% accurate.

How does cycle length affect the overall length of pregnancy?

In fact, it makes no difference how long your monthly cycle. On average, about nine months pass from conception to the date of birth, but this period may deviate in one direction or another depending on the individual characteristics of the woman, as well as on her course. specific pregnancy.

Conclusion

We talked to you about the difference between the actual and obstetric term of pregnancy. Correct definition duration of pregnancy, taking into account the length of the cycle and approximate dates ovulation, helps to calculate the approximate date of birth as accurately as possible. That is why it is extremely important to inform your doctor about all the features of your body.

Having discovered the long-awaited two stripes, the woman realizes that very soon she will have a baby, and possibly two. Arriving at an appointment with an obstetrician-gynecologist, she is interested. Very often, hearing the number in weeks, future mom is embarrassed and thinks that the doctor thought something was wrong, because sexual intimacy was at a different time (usually 2 weeks later). Don’t worry, the doctor told you the obstetric gestational age, and you, guessing when conception occurred, calculated the embryonic one. Both are true. However, all over the world for a long time Among obstetricians and gynecologists, it is customary to determine the duration of pregnancy from the first day of the last menstruation - the so-called obstetric period. There are several reasons for this:

• Firstly, this method has existed for a long time, since then, when they did not know about ovulation, there were no devices ultrasound diagnostics, and the main sign of pregnancy was the absence of menstruation.
• Secondly, each woman has her own length of the menstrual cycle. For some it is 21 days, for others it can reach 35. Also, some women do not have regular cycle. Therefore, in order not to calculate the day of ovulation in each specific case, it is easier to focus on the start date of menstruation.
• Thirdly, not every woman knows exactly when fertilization occurred. And even if she remembers the day of sexual intercourse on which conception most likely occurred, the egg and sperm may meet several days later.
• Fourthly, to accurately determine the gestational age, they still rely on other data: results ultrasound examination in the first trimester, date of fetal movement, size of the uterus.

How does the obstetric period differ from the embryonic period?

information As already mentioned, the obstetric period is determined by menstruation, and the embryonic period is determined by the date of conception. With a regular 28-day menstrual cycle, the first is longer than the second by an average of 14 days, since ovulation occurs in the middle of the cycle (and it is during this period that conception occurs). However, if a woman has an irregular menstrual cycle, the difference can be more than a month.

For example, if a woman ovulated on the 53rd day of her cycle, then the difference in this case is 53 days. This is possible with polycystic ovary syndrome, hormonal disorders, and taking certain medications.

There are also cases when a woman had her last period before her previous pregnancy. Then she breastfed, and at that time there was lactational amenorrhea (lack of menstruation). With a decrease in the amount of feeding, her egg began to mature, and then, when it met a sperm during ovulation, pregnancy occurred. IN in this case there is no need to talk about the obstetric period as such, because menstruation was more than a year back!

Obstetric term with ultrasound

Ultrasound examination is one of the ways to determine the obstetric stage of pregnancy. In some cases it is the main one (as, for example, in a situation with pregnancy against the background lactational amenorrhea or with irregular periods).

In the first trimester, the main indicator for determining the duration of pregnancy is CTE ( coccyx-parietal size) and size ovum. The doctor compares these indicators with the average and gives an opinion regarding the period. During this period, children differ least from each other in size. Subsequent ultrasounds evaluate the circumference of the fetus’s abdomen and head, and the length of its thigh. But for more later all babies, like us, are different from each other: some are taller, some are a little bigger, some have longer legs... Therefore, when comparing the results with the average, there is a large error due to physiological characteristics every child. Also here, fetal development disorders (hypotrophy, hypoxia, hemolytic disease, etc.) can make their own adjustments.

additionally The gestational age determined by ultrasound is almost no different from the obstetric one with normal developing pregnancy, since those 2 weeks from the moment of menstruation to conception have already been added to the tables of average sizes. Only when an ultrasound is performed at the very beginning (up to 4-6 weeks), the set period can coincide with the embryonic one.

Based on the above, most exact date pregnancy is one that is diagnosed before 12 weeks of pregnancy.

How to determine the obstetric gestational age

Obstetric gestational age is calculated by week, starting from the first day of the last menstruation. Based on this period, the gynecologist determines the expected date of birth and issues a certificate of incapacity for pregnancy and childbirth. In order to calculate the baby’s birthday, you can use the following formulas:

1. The last day of menstruation is 3 months + 7 days. For example, a woman’s last menstruation was on September 13th. From this date you need to subtract three months (you get June 13) and add 7 days. The expected date of birth is June 19.
2. Last day of menstruation + 9 months + 7 days. For example, September 13 + 9 months (June 13) + 7 days (resulting in June 19).
3. More the hard way: To last day add 280 days to menstruation.

And finally, it is worth noting that no matter what method is used to determine the due date of pregnancy and childbirth, nature gives us a 4-week lead time, because full-term births are considered to be those that occurred from the 38th to the 42nd week of pregnancy. And the baby himself knows best when he is born.