Tracking Pregnancy Weeks: LMP vs Conception vs Ultrasound
Published 2025-02-15 · HealthCalculators Team
Most clinicians count pregnancy weeks from the first day of your last menstrual period (LMP), not from conception. That's why a typical pregnancy is described as 40 weeks — even though the baby has only existed for about 38 weeks.
Why LMP?
The exact day of conception is rarely known, but most people remember when their last period started. LMP gives a consistent baseline for clinicians worldwide.
LMP vs conception age
Conception age (or fetal age) = LMP age − 14 days. Use our conception calculator to estimate the conception date.
What about ultrasound?
An early ultrasound (between 8 and 13 weeks) can refine the due date based on fetal size. If the ultrasound disagrees with LMP by more than 5–7 days in the first trimester, clinicians usually trust the ultrasound.
IVF pregnancies are different
For IVF, the embryo's age at transfer is precisely known. Our IVF due date calculator uses transfer date plus embryo day to give the most accurate EDD.