First day of last period 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).

Why does pregnancy start at last period?

If you were having regular periods before pregnancy, your doctor will calculate your due date based off of your last menstrual period. This goes back to the fact that in order to get pregnant, your body ovulated—or released an egg—roughly in the middle of your cycle and it was fertilized by sperm.

How do you calculate pregnancy start date?

A typical pregnancy lasts, on average, 280 days, or 40 weeks—starting with the first day of the last normal menstrual period as day 1 .Calculating a Due DateFirst, 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.

Are you pregnant from the day you conceive?

Pregnancy and Conception. Most of the time, you wont know the exact day you got pregnant. Your doctor will count the start of your pregnancy from the first day of your last menstrual period. Thats about 2 weeks ahead of when conception happens.