How accurate is a calculated due date?

Only about 5% of babies are born exactly on their calculated due date. Most arrive within roughly two weeks before or after, since the estimate is based on average timelines, not a guarantee.

Why is pregnancy counted in weeks from the last period instead of the conception date?

The last menstrual period is a known, easy-to-remember reference date, while the exact conception date usually is not. Standard dating counts from LMP even though conception typically occurs about two weeks later.

What's the most accurate way to estimate a due date?

An early first-trimester ultrasound is generally considered the most accurate dating method, since fetal size varies little between individuals at that early stage.

Pregnancy Due Date Calculator

Calculate your estimated due date and see your complete pregnancy timeline and milestones.

First Day of Last Menstrual Period

Average Cycle Length

How to use

LMP method: Enter the first day of your last menstrual period — adds 280 days (40 weeks) adjusted for cycle length.
Conception method: Enter your approximate conception date — adds 266 days (38 weeks).
Ultrasound method: Enter the date of your scan and how many weeks/days pregnant you were at the time.
The timeline shows key milestones with dates — passed milestones are highlighted.
Your current pregnancy week and trimester update automatically.
Due dates are estimates — normal delivery ranges from 37–42 weeks.

About this Pregnancy Due Date Calculator

A due date is an estimate, not a prediction of the exact delivery day — only about 5% of babies are born on their calculated due date, with most arriving within a two-week window on either side.

How the standard estimate is calculated

The most common method, Naegele's rule, adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14; for people with longer or shorter cycles, this estimate can be adjusted, or an early ultrasound measurement can provide a more individualized estimate.

Why pregnancy is measured in weeks from LMP, not conception

Conception date is usually not known precisely, but the start of the last period is a concrete, rememberable reference point. This is why standard pregnancy dating (and terms like "you're 20 weeks along") counts from LMP, which is roughly two weeks before actual conception occurred for a typical cycle.

Estimate methods compared

Last menstrual period: the standard method, but assumes a typical 28-day cycle
Conception date: more precise if known, since it removes the cycle-length assumption
Early ultrasound: generally considered the most accurate dating method, especially in the first trimester, since fetal size at that stage varies little between individuals