Pregnant Time

Or why pregnancy lasts 40 weeks?

Determining the gestational age and due date is based on the assumption that a woman has a 28-day menstrual cycle with ovulation on days 14–15 of the cycle. The duration of pregnancy is on average 280 days (40 weeks) from the beginning of the last menstruation. An obstetric month has 4 weeks or 28 days. Therefore, the duration of a normal pregnancy is 10 obstetric months. However, 280 days is the conventional number of days of pregnancy. Determining the true duration of pregnancy is difficult due to the fact that it is difficult to establish the exact date of ovulation, the time of sperm movement and fertilization, it is difficult to take into account all the characteristics of the woman’s body and predict the time the baby is ready to be born. Therefore, a baby is considered full-term if it is born between 266 and 294 days (38–42 weeks) of pregnancy.

To calculate the expected due date, use the Naegele formula - add 9 months and 7 days to the date of the first day of the last menstruation. A simplified method of these calculations is to count 3 months ago from the first day of the last menstruation and add 7 to the resulting number.

When determining the due date, it should be taken into account that ovulation does not always occur in the middle of the cycle. In addition, the length of pregnancy increases by approximately 1 day for each day of the menstrual cycle exceeding 28 days. For example, with a 35-day cycle, ovulation occurs on day 21, and then the due date will be shifted a week later.

Sometimes, when determining the duration of pregnancy and childbirth, the time of the first fetal movement is taken into account. To the date of the first movement, 5 obstetric months are added for primiparous women and 5.5 obstetric months for multiparous women and the estimated due date is obtained. It should be remembered that this sign has only an auxiliary meaning. Some mothers may feel the baby move at 16–18 weeks of pregnancy.

Objective research data helps the doctor determine the due date: measuring the length and size of the fetus, the circumference of the pregnant woman’s abdomen, the height of the uterine fundus, the degree of its excitability and other signs.

More than 10% of all pregnancies last longer than 42 weeks, of which 14% last longer than 43 weeks. A pregnancy lasting more than 42 weeks is called post-term. This condition is more often observed in primigravidas, whose age is greater or less than the average childbearing age, and in women who have had 5 pregnancies or more. The reasons for post-term pregnancy can be different. Often this is simply a hereditary feature, sometimes it is a pathology associated with hormonal imbalance, metabolism in the mother’s body, obesity, delayed biological maturation of the neuromuscular apparatus of the uterus, intrauterine growth retardation.

The absence of labor at the expected date of birth is often unfavorable for the baby. Normally, after 40 weeks of pregnancy, fetal growth slows down, and at 42 weeks it practically stops. Therefore, the so-called maturation disorder syndrome is observed in 30% of post-term infants. To stimulate labor, the expectant mother is advised to move more; if this is ineffective, special gels are now most often used, which are placed in the cervix to stimulate labor.

In turn, childbirth before the end of the normal period of intrauterine development (before 37 weeks of gestation) is considered premature. Among the total number of births, the frequency of premature births is 5–10%. As a result of premature birth, premature babies are born. Prematurity is a condition of a fetus born earlier than expected, weighing less than 2,500 g, height less than 45 cm, characterized by immaturity of organs and systems, insufficient resistance to environmental factors. Currently, in modern maternity hospitals it is possible to care for infants born after the 28th week of gestation and weighing more than 500 g.