First Day of Last Menstrual Period (LMP)
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 pregnancy 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.
But remember: It’s just as normal to deliver a week or two before or after.This is an estimated due date of when your baby is born. Babies rarely keep to an exact timetable, so your full-term pregnancy can be anywhere from 37 and 42 weeks.