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.

When can a due date be determined?

An estimated due date can be calculated by following steps 1 through 3: First, determine the first day of your last menstrual period. Next, count back 3 calendar months from that date. Lastly, add 1 year and 7 days to that date.

What is Naegele's rule and how is it used to calculate EDD?

Naegele's rule involves a simple calculation: Add seven days to the first day of your LMP and then subtract three months. For example, if your LMP was November 1, 2017: Add seven days (November 8, 2017). Subtract three months (August 8, 2017).