Please note when using our baby due date calculator, the baby’s due date is approximately 40 weeks from the first day of your last menstural period. The gestation period is 280 days from this date. Some babies are born before this date and some are after. It is only an approximation. Please seek professional advice on your baby and your own health if you suspect you are pregnant. Expectant Mothers/Expecting Mothers should be very diligent in following their doctor’s pregnancy advice.