How do you calculate day of conception?
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).
EDC by LMP is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. Gestation by LMP is calculated from the first day of the last menstrual period. Gestation by CRL is calculated: Weeks = 5.2876 + (0.1584 * Crown_Rump_Length) - (0.0007 * Crown_Rump_Length2).