Ultrasound during pregnancy: when and how many times can the study be done?

To monitor the course of pregnancy, laboratory and instrumental tests are carried out. Mandatory diagnostic methods include ultrasound examination (US). What is the diagnostic value of this method, how often and when should ultrasound be performed?

Ultrasound allows you to obtain images of internal organs and visualize them. In this case, the body is not exposed to harmful radiation, as during fluoroscopy, and pregnant women can undergo this procedure without fear. Ultrasound is a screening diagnostic method, that is, identifying abnormalities that do not manifest symptoms. The research allows:

• identify intrauterine pathologies;
• assess the dynamics of fetal development, the condition of internal organs;
• determine the duration of pregnancy;
• find out the gender of the unborn child;
• diagnose ectopic pregnancy;
• monitor the effectiveness of therapy for complications.

Ultrasound during pregnancy by week

During a normal pregnancy, screening ultrasound is recommended to be performed three times:

1. At 10-14 weeks (better at 12) - to assess the thickness of the collar space (neck fold, TVP). A deviation of this indicator from the norm is characteristic of chromosomal disorders, in particular, it indicates the likelihood of having a child with Down syndrome.
2. 20-24 weeks, to assess the condition of the internal organs of the fetus and identify congenital developmental anomalies.
3. 30-34 weeks - to assess the baby’s blood supply and identify intrauterine growth retardation.

If the pregnancy is complicated, additional examinations are prescribed.

Obstetric and embryonic stages of pregnancy

It is generally accepted that during a normal pregnancy, its average duration in the population is about 280 days

, if counted
from the first day of the last menstruation
, which is called the
“obstetric period of pregnancy”
. This calculation is used to determine the timing of prenatal leave and the expected date of birth, as well as to estimate the size of the fetus according to ultrasound data during pregnancy more than 12 weeks.

To determine the expected date of birth, according to the obstetric period, from the date of the first day of the last menstruation on the calendar, count back three months and add 7 days

.
However, it should be borne in mind that the expected due date is not a fixed date. This is just a certain period of time, ± 10-12 days, when childbirth is most likely to occur
. The fact is that the gestation period for each woman in each of her pregnancies is strictly individual, varying within certain limits around an average duration of 280 days.

Sometimes they use the calculation of the so-called “embryonic period”

pregnancy from the moment of conception, which often almost
coincides with the date of ovulation
.
The fact is that a woman’s menstrual cycle (the period from the 1st day of one menstruation to the 1st day of the next menstruation) lasts on average 28-30 days. During the first half of the menstrual cycle, a follicle matures in one of the ovaries, and ovulation occurs on days 14-15. In this case, a mature egg is released from the follicle. Taking into account that a mature egg is capable of fertilization within 2 days after ovulation, and sperm have fertilizing activity within 4 days after ejaculation, the total period of the most probable possibility of conception is about 6 days. The embryonic period is shorter than the obstetric period by approximately 14 - 15 days
.

In clinical practice, doctors mainly use the calculation of the “obstetric period”, since most pregnant women know well when the first day of the last menstruation was, but not all of them have a clear idea of ​​the date of conception.

It is also possible to determine the gestational age by the size of the uterus and the movement of the fetus. However, these criteria are not precise, as they have a pronounced individual character. The fact is that the size of the uterus at the same stages of pregnancy in different women varies within quite a wide range, which does not allow us to estimate the gestational age of each specific patient with an accuracy of up to a week.

The sensation of fetal movement is also very subjective and depends on the sensitivity threshold of each woman. So some women may, for example, begin to feel fetal movements from 18 weeks, while others only from 22 weeks. But in fact, the fetus begins to move from 7-8 weeks.

A fairly typical and widespread misconception is that the gestational age is determined using ultrasound (ultrasound).

. This doesn't actually happen. In the process of this study, another equally important question is solved - for what period of pregnancy are the fetal sizes obtained during the study typical, provided that the expected period of pregnancy is known, which, in turn, is calculated from the first day of the last menstruation or from the date of conception. Therefore, if in some observation, for example, it is known that the expected gestational age is 32 weeks, and according to ultrasound data, the size of the fetus is characteristic of a gestational age of 29 weeks, then no one will argue that the gestational age is 29 weeks and not 32. It will it was concluded that there is a delay in fetal size by 3 weeks, which will require a more in-depth examination to determine the reasons. In addition, in dynamics, ultrasound assesses the rate (speed of fetal growth) in accordance with the gestational age.

During the normal course of pregnancy, which occurs in most patients, the size of the fetus and the gestational age coincide, which creates a false assumption that ultrasound can determine the gestational age.

It is also important to take into account the following circumstances: up to 12 weeks of pregnancy, when the length of the embryo (its coccygeal-parietal size - CTE) and the average internal diameter of the fetal egg are assessed using ultrasound, the correspondence of the measurement results to the gestational age is assessed using tables calculated for the “embryonic period” ", which is 2 weeks less than obstetric.

At a later date, to compare the results of fetal measurements using ultrasound, tables calculated for the obstetric period

(from the first day of the last menstruation).

If the doctor who conducted the study up to 12 weeks did not make an adjustment by 2 weeks for the obstetric period, then discrepancies of 2 weeks may arise between the gestational ages that were suggested by ultrasound data before 12 weeks and after. In reality there is no difference. You should only add 2 weeks to the result obtained before 12 weeks, if this has not been done.

Thus, the gestational age in weeks is determined only on the basis of a set of data, including the date of the last menstruation, the date of conception, dynamic ultrasound results, and data on the rate of fetal growth.

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Types of ultrasound during pregnancy

Typically, a pelvic ultrasound is performed through the abdominal wall (transabdominal). But in the early stages there may be indications for a transvaginal (intravaginal) examination. With it, a sensor is inserted into the vagina and allows you to “examine” the internal organs at close range, in detail. Such a study is prescribed before 12 weeks if the pregnant woman is obese or has had a cesarean section, and the scar on the uterus prevents visualization of its contents.

In the later stages, along with traditional ultrasound, the following can be performed:

• Dopplerography - the study of blood vessels and blood flow - is carried out during the third screening study. It is based on the Doppler effect - the characteristics of the reflection of an ultrasound beam from moving objects, which include blood cells. Such an examination allows you to assess the adequacy of the blood supply to the fetus, the state of its circulatory system, the vessels penetrating the walls of the uterus and the umbilical cord;
• 3D/4D ultrasound is performed during the second screening study. A three-dimensional image of the fetus is created and its changes are monitored over a certain period of time. This is an informative diagnostic method that identifies developmental defects.

Determination of gestational age

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