The availability of ultrasound diagnostics allows a woman who has just found out about the onset of pregnancy not to speculate about the term, but to find out exactly how her baby is developing. Naturally, when visiting an ultrasound room, a woman roughly assumes how long the doctor will call, but it happens that her expectations are not justified.

How often does this happen, how to explain the situation when the term for ultrasound differs from the term for menstruation, or the size of the fetus differs from the standard ones at a specific time?

How reliable can ultrasound be?

Ladies who really want to get pregnant want to know about the miracle in their lives as early as possible, literally in the first days of delay. Of course, you can always buy a pregnancy test. But even supersensitive tests can only accurately give a specific answer to the question "did you get pregnant?". They are able to answer yes or no, but to set a time limit, more precise methods will be required.

Positive pregnancy test

During a manual examination of a pregnant woman, an obstetrician-gynecologist may notice that the uterus is loose and slightly enlarged. But this phenomenon is also observed before menstruation. To accurately clarify the situation, an ultrasound examination is needed.

Ultrasound is able to accurately determine pregnancy when the level of hCG (chorionic gonadotropin) in a woman's blood exceeds a thousand units. In this case, the doctor is already able to see a fetal egg in the uterine cavity (in case of multiple pregnancy, two or three fetal eggs). The longer the period, the more complete the picture shows the study - the doctor states the presence of not only the fetal egg, but also the yolk sac in it, or even see the embryo and its heartbeat, and be able to measure the KTR of the embryo (the distance from the coccyx to the crown).

If your menstrual cycle is more than 30 days, the gestational age on ultrasound and on menstruation can differ significantly.

Often this situation arises: a woman comes for an ultrasound scan, the doctor is interested in the date of the last menstruation, conducts a study, and then announces that the pregnancy, most likely, turned out to be frozen - the embryo and its heartbeat are not visualized, a fetal egg is visible, the size of which is smaller than it should be. The doctor may be wrong if the woman had a late ovulation. In a significant part of women, the menstrual cycle is not the standard 28 days, but 33, or even 35-40. This means that the conception did not occur on the 14-16th day of the cycle, but a week or two later, and the woman simply came to the ultrasound too early, so the fetus was not seen. Most likely, the pregnancy will not be frozen and the fetus is developing normally, it’s just that the study should be repeated in a couple of weeks, so as not to be mistaken. It is also possible to explain the situation when pregnancy is not visible at all on ultrasound: probably, the level of hCG in the blood is less than that at which pregnancy can be seen.

Differences in terms of pregnancy by menstruation and by ultrasound

Obstetric gestational age is calculated by weeks

All obstetrician-gynecologists use the term "obstetric term" when determining the gestational age. This allows all gynecologists in the world to speak the same language. It is conducted in weeks, the countdown of which starts from the first day of the last menstruation. The first and second obstetric weeks are those weeks during which the egg is still maturing in the body, which will subsequently come out of the ovary during ovulation and meet with the sperm.

Thus, a woman who has a standard menstrual cycle of 28 days and who comes to the doctor with a delay of one week will be diagnosed with pregnancy at exactly five obstetric weeks. At the same time, an ultrasound examination can set a slightly shorter period - three, four weeks. To what extent is this justified and why does the deadline not coincide? This situation is normal, since ultrasound aims not to determine the exact period in weeks, but to determine how many weeks from conception the fetus has developed according to its parameters (its gestational age). It is important to understand the difference between these two concepts. Therefore, a discrepancy of 1-2 weeks between the period for menstruation and ultrasound in favor of menstruation (i.e., the period for menstruation is longer, and for ultrasound it is less) is a common occurrence, there is nothing to worry about.

How to determine the gestational age with an irregular cycle?

Some women suffer from hormonal imbalances and, as a result, irregular cycles. With such pathologies as polycystic, multifollicular ovaries, an excessive amount of male sex hormones, menstruation may not come for several months. Nevertheless, ovulation with such disorders does occur from time to time, which means that such women have almost the same chances of becoming pregnant as the rest.

The girl remembers the date of the last menstruation

If pregnancy is undesirable for you, the use of contraceptives is mandatory, even if you suffer from an irregular cycle and do not have periods for several months.

Often, women with an irregular cycle do not consider it necessary to protect themselves, believing that they will definitely not be able to get pregnant. In this case, pregnancy can be detected in the third or even fourth month, because the absence of menstruation for several months becomes a kind of norm. A correct calculation of the obstetric term in such a situation is practically impossible, therefore, when determining the term, the doctor can rely solely on ultrasound data. If the fetus is not too large or too small, an ultrasound can determine the gestational age of the fetus to within 2-3 weeks. The date of birth will also be set by an unconventional method - counting 40 weeks from the first day of the last menstruation, and based on ultrasound data.

In what cases is the term for ultrasound ahead of obstetric?

Currently, in Russia, all pregnant women are given the right to undergo a free ultrasound examination three times. They are usually prescribed between 11 and 14 weeks, between 18 and 22, and between 32 and 34. Each of these ultrasounds has its own goals (the first is to determine whether gross fetal malformations and genetic abnormalities have been seen, the second is to control the development of internal organs, the third is the determination of the state of the placenta, the position of the fetus and its approximate weight). However, with each examination, the doctor determines how many weeks the fetus is developed. And often this period is ahead of obstetric. For example, a woman knows for sure that her pregnancy is 20 weeks, and the doctor indicated in the conclusion that the size of the fetus corresponds to 22 weeks. Why is there such a situation? There is no error in it. This can be explained as follows:

• The fruit is large. The large size of the fetus, ahead of its gestational age, is not a pathology. Just like all people, the fetus already in the womb has its own individual characteristics.

• The size of the fetus is slightly smaller than it should be. This may well be a variant of the norm, perhaps the baby is just small, especially if his parents are not tall and impressive in weight.
• Incorrectly defined obstetric weeks. There are cases when bleeding, which a woman takes for the next menstruation, is actually a threat of miscarriage during a pregnancy that has already occurred in the previous cycle. In the people, this phenomenon is called "washing the fetus." It turns out that a woman gives the doctor one date of the last menstruation, believing that conception occurred in this cycle, while in fact it happened in the previous one, and, accordingly, the obstetric period will be longer.

How often do ultrasounds go wrong?

There is always the possibility of an error, both up and down, when determining the term by ultrasound. Why is this happening and how many factors are to blame for this? There are several of them: outdated equipment, insufficient qualifications of a functional diagnostics doctor, confusion with the date of the last menstruation, the fetus was not immediately seen, the individual characteristics of the fetus (too large or too small). You can minimize the risk of error. It is enough to visit trusted medical centers where the level of qualification of doctors and the quality of equipment is beyond doubt, as well as carefully monitor your cycle and know exactly the date of the last menstruation.

Dear Olya!

Your assumption that the obstetric gestational age is determined by ultrasound is not entirely correct. This study is more likely not to establish the term, but to determine whether the size of the fetus corresponds to the estimated true gestational age.

Calculation of the gestational age on ultrasound

During an ultrasound examination in the early stages, fetal development indicators are evaluated according to tables calculated for the embryonic period, which is 2 weeks less than obstetric. There are several such tables compiled by different authors, which contain data on the average size of the fetus at different stages of pregnancy and suggest an error interval that can reach 2 weeks. Ultrasound machines have built-in programs in which averaged indicators taken from different tables are taken. It is possible to judge whether the fetus has a developmental delay only if the woman knows exactly when the conception occurred, otherwise the specialist can only make a preliminary conclusion, which must be confirmed by additional studies.

In the early stages of pregnancy (up to 12 weeks), experts pay attention to the length of the embryo from the coccyx to the crown (KTP) and the average internal diameter of the fetal egg. At later stages of pregnancy, the development of the fetus occurs in jumps, so the tables that the ultrasound doctor is guided by are designed for the obstetric period, that is, from the first day of the last menstruation.

It is for this reason that there are often discrepancies between the deadline set for an ultrasound scan and the deadline that passes in all the documentation drawn up in the antenatal clinic. Therefore, very often ultrasound doctors give out some average result, taking into account the data on the date of the last menstruation, the date of conception, the results of previous ultrasounds and data on the development and growth of the fetus, which in most cases coincides with the true gestational age if it proceeds normally.

How to determine the date of conception?

To determine the date of conception, you need to calculate the time at which you ovulated and analyze which of the days when you had sexual intimacy was closest to the period of ovulation. It is worth noting that it is almost impossible to determine exactly when a child was conceived, but, nevertheless, an approximate calculation can be made.

To determine when you ovulate, you need to know the length of your menstrual cycle. If it is stable and is 28 days, then ovulation most likely occurs on the 14th day of m.c. If its duration is 29 days, then on the 15th day of the m.c. With a 30-day cycle on day 16, etc. The data from the first ultrasound and the hCG level from the analysis from 7.04 indicate that you could have ovulated between March 6 and March 8, 2015. If you adhere to these data, then according to the results of subsequent ultrasound, the development of the child approximately corresponds to the embryonic gestational age with a slight excess. But, as mentioned earlier, fetal development is spasmodic, so an error of +/- 2 weeks is considered a variant of the norm.

The diagnosis of "fetal growth retardation" cannot be made solely on the basis of a difference in gestational age. Slow fetal development must be confirmed by regular measurements of the height of the fundus of the uterus and the circumference of the abdomen, which are made at each visit to the obstetrician-gynecologist; study of the level of placental hormones; CTG data (fetal cardiac cardiotocography); and ultrasound in case of delayed development reveals a low birth weight of the fetus, disproportionate growth of organs, impaired blood circulation through the vessels of the umbilical cord and arteries, and other possible deviations from the norm.

Best regards, Xenia

The gestational age is an important parameter on the basis of which the growth and development of the fetus is analyzed, and the expected date of birth is also calculated. The difference in numbers can lead to bewilderment and confusion for both expectant mothers and doctors. It is possible to determine the gestational age according to various parameters: the results of an ultrasound examination, the size of the uterus during a vaginal examination, movement, but most often the main reference point is menstruation. What does it mean if these numbers do not match, for example, the period for ultrasound is longer than for menstruation? When should a expectant mother worry? Can a doctor make a mistake and how to calculate correctly?

Read in this article

Causes of differences in obstetric terms and monthly

For a more thorough understanding, it is necessary to understand on the basis of what parameters it is possible to determine the gestational age.

• The most significant guide for which you need to know the first day of the onset of the last spotting. If a woman remembers this date exactly, then in all situations this parameter is preferred.
• The gestational age can also be determined by ultrasound examination (ultrasound) of the fetus. For the entire period of gestation, a woman performs it at least three times. The most informative for the setting of the term is an ultrasound examination at 11-13 weeks. The main measured parameter is KTR (the distance from the coccyx to the crown of the embryo). Age is determined by its value in weeks. Sometimes ultrasound is performed at a very short time, when visualization of even the embryo itself is difficult, and only the fetal egg is determined. It happens that some “not sharpened” devices for obstetrics can give out a period from the moment of ovulation. In such a situation, of course, the period for menstruation and ultrasound does not coincide. The difference will be on average 2-3 weeks. As a rule, obstetrician-gynecologists are aware of such nuances of research, and take this into account when determining the number of weeks of pregnancy.
• An important parameter is the determination of the degree of enlargement of the uterus during the initial examination of a woman by a gynecologist. Experienced doctors can tell the time to within a few days. But with each month of pregnancy, the error in setting the term increases. The most informative such study is from 7 to 16 weeks.
• Also, the gestational age can be determined by the beginning of fetal movements. On average, a woman feels the beginning of movements during her first pregnancy at 20-22 weeks, with a second pregnancy at 18-20. But the unborn baby begins to move actively from the second month. Therefore, very often women note that even at 14-16 weeks they distinguish the movement of their crumbs. That is why this study gives a very approximate and subjective result. In such situations, there may also be a discrepancy in the gestational age.

It becomes clear that you have to take into account all the nuances and determine the number of weeks of gestation by all possible methods. With significant deviations, the average period is displayed with an emphasis on the indicator for menstruation or ultrasound. Why might there be differences? Consider the most popular situations when the numbers will differ.

Short or long menstrual cycle

Conception occurs during ovulation, usually on the 14th - 16th day. In women whose cycle length is more or less than the standard 28 days, the release of the egg from the ovary is shifted. Ovulation can occur at the beginning (even on the 3rd - 5th day of critical days) or on the eve of menstruation. Accordingly, the absence of a pattern leads to the fact that the gestational age in terms of the size of the fetus (this is determined by ultrasound) may not correspond to similar figures for menstruation. But, as a rule, such a run is no more than 2 - 3 weeks and can be traced throughout the pregnancy.

With a short or long cycle, women are often taken for menstruation at the earliest possible date. The discharge, of course, differs from ordinary menstruation in the number of days and profusion, but this is not always noticed.

irregular periods

Often, the period for ultrasound is less than for menstruation, if the woman’s cycle is completely irregular, and the interval between critical days is more than 35 days. This happens with PCOS (polycystic ovary syndrome), endocrine diseases, pathology of the pituitary and hypothalamus, etc. If the pregnancy is planned, then the girl can purposefully “catch” ovulation, which is not so easy to do in such a situation. The most convenient way is to build a graph, in addition to which you can determine the release of an egg from the ovary. If the doctor knows the approximate date of ovulation, then it will be easier to compare all the dates and derive the most accurate one.

Loop failure

Pregnancy is also possible against the background. This can be either a single failure due to external factors, or induced, for example, while taking hormonal contraceptives. Girls, using emergency contraception, for example, or, believe that unprotected sex immediately after taking the pills will also not lead to pregnancy. In fact, these hormonal drugs only prevent the possible conception that could have happened within 48 to 72 hours before taking the remedy.

Failure to follow the rules for taking oral contraceptives - skipping a pill, changing the time - can lead to a failure of the menstrual cycle and pregnancy. In such a situation, there is also often a difference in the period for menstruation, ultrasound and other parameters.

Cycle failure can occur against the background of stress and psycho-emotional experiences, as well as with an increase in body weight. If conception occurs just for such a period, then differences in the established dates may be observed.

Lactation

During breastfeeding, especially during the first year, the process of releasing an egg may not exactly match the cyclical nature of the menstrual cycle. As a result, when calculating the period by monthly, a big error is made in terms, sometimes up to 3-4 weeks.

Intrauterine interventions

Any manipulation, accompanied by penetration into the uterine cavity, can cause a violation of the menstrual cycle. Also, after them, for another month or two, irregular spotting is often observed, which a woman can falsely take for menstruation. This can happen after diagnostic curettage, hysteroscopy, abortion (including medication), etc.

When using assistive techniques for conception

Often you have to resort to ovulation stimulation in a woman, the creation of an artificial hormonal background in order for conception to occur. In such cases, there may also be a difference between ultrasound and menstruation. Especially if IVF (in vitro fertilization) was used. In such situations, it is difficult for women to figure out the timing on their own, since the eggs are first taken, which are fertilized outside the woman's body. And only after that, on the 21st day of the cycle, the embryos are replanted. As a rule, the difference in terms for ultrasound and menstruation will be the same for all studies, for example, at two or three weeks.

If there was a threat of interruption in the early term

Early pregnancy may be accompanied by various kinds of bloody discharge, which a woman can take for menstruation. In fact, these are the first symptoms of an interrupt threat. This can last up to two to three months, during which on the expected critical days bloody daub or even moderate discharge will appear. In such situations, the difference in the figures of the period between the monthly and data obtained by other methods will be immediately interpreted correctly with a thorough history taking.

Development of a large fetus

In the case of the development of a large fetus in utero, a false impression is created that pregnancy by ultrasound is greater than by menstruation. However, such a difference will be visible starting from 20 to 22 weeks. Often only certain parameters of the baby are ahead, for example, the circumference of the abdomen or the length of the femur. At 11 - 13 weeks, the gestational age according to ultrasound should correspond to others.

Permissible difference

Determining the term is important not only for determining the expected date of birth (PDR), but also for assessing the growth rate of the baby in utero. For example, a developmental delay of more than 4 to 6 weeks is an indication for emergency delivery at any time during gestation.

A delay of up to two weeks is acceptable. It is also not critical if the difference can be traced from the very beginning of pregnancy. This indicates that errors were initially made in the definition.

If the period for ultrasound exceeds the period for menstruation, this has a more favorable prognosis than the reverse situation. In case of lagging, you should make sure that the fetus does not suffer in utero, it has enough nutrients and oxygen, and the placenta functions in full.

How long to focus on childbirth

Both doctors and women often face the question of how to determine pdr - by ultrasound or by menstruation. The approach is individual in each situation.

If the expectant mother clearly remembers the date of the last menstruation, her cycle is regular and there have been no special changes in her lifestyle lately, then menstruation is preferred.

In a situation where a woman hardly remembers the last critical days, or her cycle is irregular, its duration is more than 35 days, then you should be guided by the ultrasound of the fetus. Moreover, all the dates of its implementation are taken into account, compared, and only after that the estimated date of birth is determined.

Why are weeks of pregnancy counted?

Everyone knows that the duration of pregnancy is nine months. However, this is a very average figure. For a more thorough monitoring of a woman "in position", for monitoring the growth and development of the fetus, terms in weeks and days are used. So, full-term pregnancy is considered from 37 to 42 weeks of gestation, which ranges from 259 to 294 days. A child born in this interval has completely adapted organ systems for life in the Earth's atmosphere (of course, if the baby does not have a congenital pathology or infection).

Here's why you need to know the exact gestational age:

• Timely identify the pathology at the rate of maturation of the baby and carry out the necessary treatment. Sometimes it is even premature delivery, when the continued presence of the baby in utero has a high risk for his life.
• To systematize the clinical examination of pregnant women and the timing of registration. Based on the results, possible complications are evaluated, and measures are taken to prevent them.
• Knowing the gestational age, you can set the deadline for the expected date of birth. Overwearing can adversely affect the health of the baby, in which case labor can be induced.

Is the time determined by ultrasound accurate?

During pregnancy, ultrasound is performed several times. What timeframe is more accurate? First of all, the results of ultrasound obtained from the 11th to the 13th week, inclusive, should be adopted. It is during this period that the dimensions of the crumbs are such that it is possible to accurately determine the reference points for measurement (as a rule, this is the KTP - the distance from the coccyx to the top of the parietal bone on the head of the embryo), which can be displayed on the screen at once.

In the short term, the parameters are approximate, since often only the fetal egg is visible or the structures of the unborn baby are not clearly distinguishable.

After 16 - 18 weeks, only individual parts of the fetus can be displayed on the monitor - the head, arm, leg or tummy. These structures are measured and the approximate gestational age is determined from the summary tables. The longer the period, the less indicative the numbers are, since there is still someone thin in the mother’s stomach, while others have chubby cheeks and legs.

Determining the gestational age has an important diagnostic and prognostic value for the successful management of a woman during childbearing and preparing her for childbirth. That is why every girl should strictly maintain a menstrual calendar and mark in it all significant health events. This is a great help to doctors, help in solving emerging problems. Of course, it doesn’t matter whether they gave birth by ultrasound or by menstruation, the main thing is that mom and baby are healthy and nothing threatens their lives. And this can only be achieved through coordinated work between doctors and women.

A woman who finds herself in a position in most cases is interested in the question of which of the two is true: the period for menstruation or the period for ultrasound. And if experienced women do not experience problems with determining the age of the fetus, then for the first time pregnant women do not have a clear idea about the differences between obstetric and gestational periods.

When diagnosing pregnancy, the gynecologist voices the obstetric period in weeks. An important feature is that the reporting point is the first day of the menstrual cycle. As you know, conception occurs during the period of ovulation (about day 14). In this scenario, in fact, the woman is not yet pregnant at the moment when menstruation begins. That is why, in most cases, the approximate date of birth (DDR) differs by 2 weeks from the real one or the one set by ultrasound in a smaller direction.

But it is this method that is optimal and is used in obstetric practice. This is true, because the egg begins its development on the first day of menstruation, and then matures and is fertilized, and if not, it dies. Therefore, the obstetric period can be considered the "age" of the egg. Also, menstrual cycles are individual and can vary greatly from woman to woman. Although the 28-day menstrual cycle is considered to be a reference, the actual values ​​​​can vary greatly.

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So, many women may have a cycle of more than 28 days, for example, 35. In this case, ovulation occurs on his 16-17th day. Accordingly, if the cycle is less, for example, 21 days, then the release of the egg from the body of the ovary takes place on the 10-11th day. To simplify the work of specialists, it is customary to consider the beginning of pregnancy from the first day of the last menstruation, which is called the obstetric period.

According to the results of ultrasound examination

In the case of determining the maturity of the fetus according to the results of an ultrasound examination, a controversial situation may arise:

1. The temporary course of pregnancy by ultrasound is determined by assessing the development of the fetus, metric indicators, the condition of the uterus and the placental barrier (in the II and III trimesters). One of the indicators is KTP (coccyx-parietal size), which is almost the same for different fetuses at the initial stages of development. Pay attention to the size of the fetal egg in the first 12 weeks of gestation. The most accurate is the number of weeks established by ultrasound in the first trimester of pregnancy (up to 12 weeks). After the indicators may vary up or down due to the individual characteristics of the development of the unborn child.
2. As a rule, the embryonic period determined by ultrasound indicates the period from the moment of fertilization of the egg by male material to the present, therefore it is considered to be true in fact. Most often there are discrepancies of about 2 weeks between the EDD by ultrasound or calculated by monthly. But disability certificates and other documents are issued based on the obstetric age of the fetus, which is indicated in the mother's passport and documentation in the antenatal clinic.

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Conclusion and Conclusions

Be that as it may, it is almost impossible to establish exactly when the child was conceived and when he will be born. Even knowing the exact date of the “decisive” sexual intercourse, one cannot be sure that fertilization took place on that day, since spermatozoa can exist for 24 hours. Moreover, there are many factors that provoke labor activity earlier than expected.

If we consider the question of the correctness of the period for menstruation or ultrasound, it should be said that both are correct, but traditionally obstetricians focus on the first option. This avoids confusion in the future and considers a possible delivery on the indicated dates. But, according to statistics, not every woman gives birth on the day established by specialists. It is considered normal to have a run of 4 weeks (from 38 to 42 weeks of gestation, respectively).