Pregnancy Calculator

From your last menstrual period (LMP), it adds 280 days — this is Naegele's Rule, recognized by ACOG. From a known conception date, it adds 266 days instead, since conception happens roughly two weeks after LMP. Your current week (gestational age) is simply today's date minus your LMP date, converted to weeks and days.

Because the first day of your last period is the one date most people actually know with confidence — ovulation and conception dates are harder to pin down without tracking. Counting from LMP means gestational age reads about two weeks ahead of your actual conception date, which is a convention, not an error.

Naegele's Rule assumes a 28-day cycle with ovulation on day 14, so this calculator shifts the estimate to match your entered cycle length — a 35-day cycle moves the due date about 7 days later than the default math would give. If your cycles are irregular or you're unsure of your length, LMP-based dating becomes less reliable, and an early ultrasound is the better source of truth.

It's an estimate, not a prediction — ACOG notes only about 4 in 100 babies are born on their exact due date, though most arrive within roughly two weeks either side. A first-trimester ultrasound dates a pregnancy more precisely than LMP-based math. If your provider's scan date and this estimate differ by more than about a week, ACOG guidance is to go with the scan date.

They're general, commonly cited windows used for prenatal scheduling and fetal development — things like a detectable heartbeat around 6-8 weeks, the anatomy scan around 18-22 weeks, and viability around 24 weeks. They're meant as rough orientation, not a personalized schedule — your own provider's prenatal calendar may differ and should take priority.