Obstetrician-gynecologists consider the date of the end of the 40th week of pregnancy to be the expected date of birth. Calculate this date in several ways.
The most common is a calculation based on the date of the first day of the last menstruation. This is the first question an obstetrician-gynecologist asks you when you come to an appointment, and in our case the most important one.
280 days (9 lunar and 10 obstetric months) are added to the date you announced – the date of birth is obtained. It can be calculated using the Negele formula (named after the German obstetrician Franz Karl Negele, who proposed this calculation system in the 19th century): from the first day of the last menstruation take three months and add seven days. Electronic pregnancy calendars that are popular now are programmed on the same principle: the first day of the last menstruation is taken as a starting point.
But it is important to understand that this method is only suitable for women with a regular menstrual cycle.
Some physicians in calculations are also based on the date of ovulation. It is assumed that the viability of the egg retains during the day – respectively, the date of ovulation is the date of conception. But the date of ovulation varies depending on the duration of the menstrual cycle (from 21 to 35 days).
The average value – 28 days – is observed only in 52% of women. Depending on the length of the menstrual cycle, the time of ovulation also changes: conception may occur within the 8–20th day of the cycle.
When the 28-day menstrual cycle, the egg from the ovary, as a rule, occurs on the 14−15th day. But even here changes are possible – under the influence of stress, climate change, infectious diseases.
With such a calculation, it is necessary to remember that the duration of pregnancy will be 38 weeks, or 266 days, that is, the date of birth in this case is the end of the 38th week of pregnancy (calculated from the expected date of ovulation).
But to make calculations based on the exact date of sexual intercourse, it is not necessary. After all, pregnancy can occur not only when sexual intercourse occurred on the day of ovulation, but also if it happened 2–3 days before ovulation (during this time the spermatozoa retain their viability).
But it is worth remembering that even with an ideally stable menstrual cycle with such calculations, the date turns out to be less than that of 10% of women. For the rest, childbirth occurs either before this time (full-term pregnancy is considered from the 38th week counting from the first day of the last menstruation) or later, at the 41–42th week.
The calculation described above is added to the methods described above based on the day when the expectant mother first felt the movements of the baby. Those who are preparing to become mothers for the first time, feel the movement of the baby on the 18-20th week, mothers with experience – a couple of weeks earlier. But the sensitivity of the organism as a whole and the walls of the uterus in particular varies – and someone may not feel any movement of the crumbs until the 22nd – 23rd week, and someone very early confuses the movements of the crumbs with intestinal motility.
So this method can only be considered as additional.
The term obstetrician-gynecologist can presumably be called the birth date already at the first examination of the future mother. And the sooner she turns to the doctor, the more precisely the time will be set. The main criteria for determining it is the size of the uterus.
At 4–5 weeks, the uterus reaches the size of a chicken egg, and at 12 weeks, the head of a newborn. In later periods, the doctor will be based on data from the height of the bottom of the uterus.
The bottom of the uterus is the uppermost part of it, and at 14-15 weeks it is located midway between the pubic bone and the navel. Of course, this method is not reliable, but it was widely used before the advent of ultrasound diagnostics, and obstetrician-gynecologists with experience in observing a pregnant woman and feeling the abdomen can calculate the date of birth very accurately.
But progress does not stand still, and ultrasound diagnostics in the matter of determining the duration of pregnancy and, accordingly, the date of expected birth comes to the fore today. Ultrasound significantly improves the accuracy of determining the duration of pregnancy.
Up to 12 weeks, it is calculated on the basis of the CTE — the coccyx parietal size of the embryo. Each period of pregnancy corresponds to a specific KTR.
Based on the results of ultrasound conducted in the first trimester, the date of delivery can be determined with an accuracy of 1-2 days.
In the second and third trimesters, the baby measures the transverse size of the head, head circumference, average diameter of the chest and abdomen, abdominal circumference, and length of the femur. The computer processes this data and calculates the duration of the pregnancy and the expected date of birth. But for periods of 13-28 weeks, the error in determining the date of delivery can be up to 7 days.
In the third trimester, however, it is difficult to calculate the date of birth according to the ultrasound scan. The fact is that the size of the child most accurately corresponds to the period of pregnancy only in the first trimester.
In II, deviations are possible, and in III there are already significant differences in the data. After all, full-term babies are born with different body weight and height.
In the pediatric ward of any maternity hospital you can see two babies born at exactly 40 weeks of obstetric time and having a weight of 2,900 g and 4,100 g and a height of 50 cm and 54 cm, respectively. That is why the examinations regularly conducted by an obstetrician-gynecologist manually do not lose their relevance even with the modern development and dissemination of ultrasound diagnostics – they complement the data of computer calculations. It is worth paying attention also to the fact that ultrasound diagnostics in 3D will not give more accurate data on the size of the baby and the gestational age compared to the standard format.
Ultrasound in 3D allows only a closer look at the organs of the baby in case of suspicion of any developmental pathologies, as well as showing the child to parents “in all its glory.”