Transcript: Congratulations-you’re pregnant! But how will you know when to expect your bundle of joy? Once your pregnancy has been confirmed, what you most want to know is your due date. Luckily, calculating your baby’s due date is pretty easy to do at home.   The average length of a pregnancy is 40 weeks, or 280 days, from the FIRST day of the LAST menstrual period, or LMP. Calculating the due date, or expected date of delivery, for a pregnancy is quite simple, then, if you know that date. Simply add nine months and seven days to the date, and you've got your pregnancy due date.Here's an example of how it works:  say the first day of the last normal menstrual period was February 1st. Add seven days to that number, and you get February 8th.  Add nine months, and you get November.  The expected due date of that pregnancy, then, is November 8.Many factors play into determining the exact day you may have conceived.  If your cycle length is irregular, you may not be able to use the first day of your last menstrual period as a marker. And similarly, if you have no idea when your last period was, don’t worry!In these types of circumstances, your doctor will likely use an early ultrasound to measure the fetus’ size and thus determine its gestational age. You may be wondering why you use your period, and not the day you CONCEIVED the baby, as a starting point.  This is because it’s rarely possible to gauge the PRECISE day that you got pregnant. After all, your ovulation schedule may be unpredictable, and sperm can live inside you for a few days. Remember, though, that your baby’s estimated due date is just that-an estimate. In fact, only 5% of babies make their debut on the EXACT day predicted for their birth! So don’t worry if your baby is born before or after that big X on your calendar.