Pregnancy Due Date Calculator

What does this Pregnancy Due Date Calculator do?

This calculator estimates your baby's due date and current week of pregnancy from the first day of your last menstrual period (LMP), using the standard obstetric convention of a 280-day (40-week) pregnancy.

The formula

Due date = LMP + 280 days (Naegele's rule). Current week = number of days since LMP ÷ 7, rounded down.

Step-by-step example

If your last period started on January 1st, adding 280 days lands the estimated due date around October 8th. If today is 70 days after that LMP date, you're at day 70 ÷ 7 = week 10 of pregnancy.

Tips & common mistakes

• Naegele's rule assumes a standard 28-day cycle — if your cycle is meaningfully longer or shorter, your actual due date may differ from this estimate.
• An ultrasound dating scan, typically done early in pregnancy, is generally considered more accurate than LMP-based calculation alone — use this calculator as a starting estimate, not a replacement for medical dating.
• "Weeks pregnant" counts from the LMP, not from conception — conception typically occurs about two weeks after the LMP, which is why week 2 is often called "week 0" of actual fetal development in some contexts.

Why due dates are estimates, not guarantees

Only a small percentage of babies are actually born on their exact calculated due date — most arrive within a window of about two weeks before to two weeks after. The 280-day calculation is a statistically useful planning estimate based on average pregnancy length, not a precise prediction for any individual pregnancy.

LMP dating versus ultrasound dating

Early in pregnancy, doctors often use an ultrasound measurement (typically crown-rump length) to estimate gestational age, which can be more accurate than LMP-based dating alone — particularly for people with irregular menstrual cycles, where the LMP method is less reliable.

Gestational age versus fetal age

Gestational age (what this calculator estimates) is counted from the first day of the last menstrual period, while fetal age (or conceptional age) is counted from actual conception, which typically occurs about two weeks later. This two-week offset is a common source of confusion — a pregnancy described as "6 weeks" gestational is roughly "4 weeks" from conception.

What happens at key weeks of pregnancy

Certain weeks carry particular medical significance: around week 12 marks the end of the first trimester and often the first major ultrasound; around week 20 is typically when a detailed anatomy scan is done; and "full term" is generally considered 39–40 weeks, though babies born anywhere from 37 weeks onward are usually considered term rather than premature.

Irregular cycles and due date accuracy

Naegele's rule assumes ovulation occurs around day 14 of a standard 28-day cycle. For people with longer or shorter cycles, actual ovulation — and therefore actual conception — happens on a different day, which is exactly why an early ultrasound dating scan is generally considered more reliable than LMP-based calculation for anyone with irregular cycles.

Frequently asked questions