40 weeks from the first day of your last menstrual period, not 9 months, sorry, and of course your baby rarely comes on your due date sometimes labor can ensue as much as a week and half after your due date. After 41.5 weeks, your risk of problems with your child goes up so induction of labor must occur. Luckily, by this time most women either already went into labor or are so dilated that induction and delivery goes well. If you have irregular periods, your due date may vary as this effects the day you ovulated.