Estimated Birth Date Self-calculation Last menstrual day + 280 days The average period from fertilization to childbirth is 266 days. The first moment of pregnancy should be viewed as the date when the egg and sperm meet and fertilize or implant, but since it is not known exactly, it is assumed that fertilization takes place two weeks after the start of menstruation and 280 days after the start of the last period. Calculate the due date. 14 days from the last menstruation start to ovulation, 14 days from the ovulation to the next menstruation cycle is calculated based on the 28-day menstrual cycle. If the menstrual cycle is 25 days, 277 days after 28 days minus 3 days, the person with a 35-day menstruation period is 287 days plus 7 days will be his due date. However, even if you are pregnant normally, the delayed birth without analgesia is 10% of the total pregnancy by two weeks after the scheduled date. Negel formula If you cannot subtract or subtract 3 from your last menstrua...