If the period is very regular and is once every 28 days, the period of pregnancy and delivery is 280 days from the first day of the menstruation. If the period is once every 28 days, ovulation is on the 14th day of menstruation, and fertilization occurs at this time, so if you calculate from the time of fertilization to the expected date of birth, it is 38 weeks. However, births in the first 2 weeks and the second 2 weeks of pregnancy and delivery are normal ranges. If you usually have irregular periods, you should calculate the time of pregnancy according to the time of the first ultrasound. If you calculate from the time of fertilization, more than 40 weeks is an overdue pregnancy and you need to end the pregnancy immediately.