Doctors use the start of your last period as the beginning of your pregnancy because knowing when your baby was conceived is hard to pinpoint. Even if you know the date you had sexual intercourse, it may not be the day you conceived your baby because sperm can live in the womb for three to five days.

Ovulation happens when the egg is released from the ovary, and few women ovulate on the same day each month. Combine this with the sperm’s life cycle and knowing exactly when you’re pregnant is tricky at best.