Is pregnancy calculated from the day of the menstrual period?

Patient's question:

No

Doctor's answer:

The normal gestation period is approximately 40 weeks. In medicine, the first day of the last menstrual period is often used as the starting point for calculating the estimated due date. The normal pregnancy duration is about 265 days (calculated from the day of fertilization). Ovulation occurs in the middle of the menstrual cycle, so a 40-week (280-day) pregnancy is two weeks longer than the actual time of fertilization. If a woman's menstrual cycle is 28 days long, then a 280-day pregnancy is equivalent to ten months of pregnancy (28 days per menstrual cycle) or ten menstrual cycles. This is why there is the saying "ten months of carrying a child."