How to Calculate the Days to Pregnancy and How to Predict Your Baby\’s Date of Birth 1. How to Calculate the Days to Pregnancy The calculation method of the days to pregnancy is actually based on the date of ovulation. The average female cycle is about 28 days, and ovulation occurs around the 14th day of menstruation. If a woman’s menstrual cycle lasts 28 days, her ovulation period will be on the 1st