Most babies are born about 38 weeks after conception. Since many women ovulate (release an egg that may then be fertilised) and conceive about 2 weeks after their last period, this is often about 40 weeks since the beginning of their last period. That’s why people often talk about pregnancy lasting for 40 weeks. Women with a regular 28-day cycle can calculate an estimated due date for their baby by counting 40 weeks from the first day of their last period. This may not be so simple or accurate in other situations, such as if you have long or irregular cycles, don’t remember when you had your last period, or if you became pregnant while taking contraception that affected your cycle. Use the due date calculator to calculate your estimated due date. If you’re not sure when you conceived, your doctor or midwife may refer you for a dating scan that uses ultrasound to estimate your due date based on your baby’s size.