You can use this calculator to determine an approximate date your baby will be born. Don't go printing those dated birth announcements just yet, though. Since it's not always possible to determine the exact date of conception, most due date calculators, such as this one, are based on a gestational period of 40 weeks from the first day of a woman's last menstrual period (LMP). This assumes a woman ovulates and conceives exactly two weeks after the first day of her LMP and carries the baby for 38 weeks. The problem is, of course, that this is an estimate, and 40 weeks is only an average term of pregnancy.
A normal pregnancy can last anywhere between 38 and 42 weeks. Many obstetricians will use what is called "Naegle's Rule" to determine the approximate due date: Take the first day of your last period, and count back three months, then add seven days. For example, a LMP of January 1, 2001 would give you an approximate due date of October 8, 2001 (an even 40 weeks ahead).
