Choose calculation method
Most pregnancies last around 40 weeks (or 38 weeks from conception), so typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). Another way to do it is to subtract three months from the first day of your last period and add seven days. So if your last period started on April 11, you'd count back three months to January 11 and then add seven days, which means your due date would be January 18. This is how your doctor will estimate your due date — and it’s a pretty solid target. But remember: It’s just as normal to deliver a week or two before or after
Calculating your due date based on the first day of your last period works well for women who have a relatively regular menstrual cycle. But if your cycle is irregular, the LMP method may not work for you. Because a reliable estimated date of delivery (EDD) is important, you and your practitioner can use your conception date instead if you remember it. Just add 266 days to get your estimated due date.
If you are part of the growing tribe of IVF moms, you can calculate your due date more precisely using your IVF transfer date.