Why is pregnancy calculated from LMP instead of conception?
Because the exact date of conception is rarely known with certainty, whereas the first day of the last period is an easily observable event.
Estimated due date from the first day of your last period.
Estimates expected date of delivery (EDD) and gestational age based on the first day of the last menstrual period (LMP) using Naegele's Rule.
EDD (Naegele's Rule) = First Day of LMP + 1 Year - 3 Months + 7 Days (280 days total)
Assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. Adjusts for shorter or longer cycle lengths.
Inputs: LMP = May 10, 2026 | Cycle Length = 28 Days
Summary: LMP = May 10, 2026 | Estimated Due Date = February 17, 2027 | Full Term Milestone = 37-42 Weeks
Because the exact date of conception is rarely known with certainty, whereas the first day of the last period is an easily observable event.
A first-trimester ultrasound (measuring crown-rump length) is the most accurate dating method; doctors adjust due dates if it differs by > 5-7 days from LMP.
A pregnancy is considered full term between 39 weeks 0 days and 40 weeks 6 days; early term is 37-38 weeks.