The gestational age in obstetrics can be considered in three different ways:
• by trimesters (the duration of each is 13 weeks);
• by lunar or obstetric months (each such month lasts 28 days);
• by weeks (there are an average of 40).
The countdown of the period by weeks allows you to more accurately track the condition of the mother and the development of the fetus throughout the pregnancy. Thanks to this principle, the gynecologist conducting the observation can calculate the estimated date of birth (PDR). Thus, the entire pregnancy lasts 40 weeks or 280 days, and fertilization in most cases occurs during the period of ovulation - the 12-14th day after the end of menstruation. To calculate the EDD, 280 days are added to the first day of the last menstrual cycle. For example, if a woman had the first day of her last menstruation on August 10, then the EDD will be set on May 17.
Let's explain a little easier. Suppose a woman is sure of the date of conception of a baby. She comes to the doctor to confirm the fact of pregnancy, and he calls another date - 2-3 weeks earlier. This means that the countdown is from the moment when the body has just begun to prepare for pregnancy - we are talking about obstetric weeks. There are just 40 of them. But if we count the period from the actual date of conception, these are already embryonic weeks of pregnancy, and there are normally 38 of them.