There are two types of pregnancy: foetal and obstetric. They differ in the date, which is taken for the beginning of the term.
In the case of the embryonic period of pregnancy, it's simple: the expected due date is counted from the moment of conception. The length of such a term – 38 weeks. The only difficulty of this method lies in the fact that a woman rarely can with a 100% guarantee to tell when conception took place.
Even if sex woman rare, and she can remember a specific date, it does nothing. Because from the point of view of biology, the sperm can fertilize an egg for several days after being hit in the female body. But because this method of calculating pregnancy is not ideal.
The obstetric pregnancy is used in all tables, calculations and regulations related to the fetus and that it, you will have to ask for pregnancy. At that time, you will be given a letter about maternity leave and childbirth.
Obstetric length of term – 40 weeks.