The average pregnancy lasts 40 weeks, which equals 28 obstetric months (duration 28 days), or 280 days. In reality, these terms are stacked in 9 calendar months plus seven days. So to talk about the 9 months of pregnancy. The birth of a baby at 38-42 week is considered normal if no signs of immaturity or overripe baby. If the child was born later, the pregnancy is considered post-term, otherwise premature.
Obstetricians start counting pregnancy from the first day of the last menstrual period. Usually the real a period of less than 13 to 16 days, because the conception occurs approximately in the middle of the menstrual cycle, i.e. at the time of ovulation. However, in order to avoid confusion, the doctors start counting pregnancy for 2 weeks before the date of conception.
If you don't know the exact date of conception, the right to count the weeks of pregnancy based on ultrasound. The planned study is carried out at 12-13 weeks of pregnancy. Usually it coincides with a schedule established at the first reception at the gynecologist. But if there are differences, it is better to focus on the deadline set by using ultrasound.
In some cases there may be a discrepancy between the size of the fetus in pregnancy. For example, mnogonogie, the presence of tumors, etc. These data are considered experts in the study. In this case, put the number of weeks of pregnancy calculated from the first day of the last menstrual period and by ultrasound at a later date correct it.