Pregnancy due date calculator

Common questions

Due dates and gestational age, explained

What is Naegele's rule?

Naegele's rule is the classic method for estimating a due date from the last menstrual period (LMP). Take the first day of the LMP, add 1 year, subtract 3 months, and add 7 days. That result is the estimated due date (EDD). It is equivalent to adding 280 days (40 weeks) to the LMP and assumes a regular 28-day menstrual cycle.

How is the due date estimated from the LMP?

This calculator adds 280 days to the first day of the last menstrual period, which is the reliable, unambiguous form of Naegele's rule (LMP + 280 days = 40 weeks). For example, an LMP of January 1, 2024 gives an estimated due date of October 8, 2024 by Naegele's rule (+1 year → Jan 1 2025, −3 months → Oct 1 2024, +7 days → Oct 8 2024).

What does gestational age mean and how is it written?

Gestational age (GA) is how far along a pregnancy is, counted from the first day of the last menstrual period — not from conception. It is written as completed weeks plus extra days, so 14 weeks and 3 days is written '14+3'. To find it, count the days from the LMP to the date in question, then divide by 7: the whole-number weeks plus the remainder days give the weeks+days form.

Why does it assume a 28-day cycle, and why is it only an estimate?

Naegele's rule assumes ovulation around day 14 of a regular 28-day cycle. Because cycle length and ovulation timing vary from person to person, the calculated due date is an estimate — most babies are not born exactly on it. Early ultrasound dating is often used to confirm or adjust the due date, and the ultrasound estimate may take priority when the two disagree.

What does full-term mean?

Full-term generally refers to 37 to 42 completed weeks of gestation, with 40 weeks being the estimated due date. Birth before 37 weeks is considered preterm, and pregnancies continuing past 42 weeks are considered post-term. These ranges are why gestational age is tracked in weeks+days throughout pregnancy.

Does it work if my cycles are irregular? Is this calculator free?

It still calculates a due date, but the estimate is less accurate when cycles are irregular or longer or shorter than 28 days, because ovulation may not fall near day 14. In those cases ultrasound dating is especially useful. This calculator is completely free, needs no account, and runs entirely in your browser — it is a study and practice aid, not a substitute for prenatal care or a clinician's dating.