By the date of your last menstruation, you can find out the duration of your pregnancy and the expected date of birth. All other calculations are carried out on this day. You need to subtract three calendar months from it and add seven days to the resulting amount. The number you get is the day of the probable date of birth.
For example, your last period was on March 13th. We subtract three months from this date – it turns out December 13th. We add seven days to this value, i.e. It turns out December 20th - this will be the probable day of your birth.
But there is one small nuance - all these calculations apply only to those women who have a stable menstrual cycle (28 days). Any hormonal disorders or stress affect the frequency of menstruation, the cycle shifts, and therefore it is more difficult to calculate the date of birth.