Due Date Calculator

Two hundred and eighty days from the last period

A last menstrual period beginning 1 January 2024 gives an estimated due date of 7 October 2024 — exactly 280 days, or 40 weeks, later.

The striking thing about that calculation is what it counts from. Pregnancy is dated from the first day of the last menstrual period, not from conception. Since ovulation typically occurs around day 14 of a 28-day cycle, you are counted as two weeks pregnant at the moment of conception, and the "40 weeks" of pregnancy contains roughly 38 weeks of actual fetal development.

This convention exists for a practical reason: the date of a period is something most people can report, while the date of conception usually is not.

Naegele's rule

The standard method, formalised by the German obstetrician Franz Naegele in the early nineteenth century:

EDD = first day of LMP + 1 year − 3 months + 7 days

which is equivalent to LMP + 280 days.

Working the example: 1 January 2024 + 1 year = 1 January 2025, − 3 months = 1 October 2024, + 7 days = 7 October 2024.

The rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles it needs adjusting — a consistent 32-day cycle shifts ovulation about four days later, and the due date with it. This calculator takes cycle length as an input for that reason.

Where conception date is known — from IVF, a documented ovulation test, or a single known intercourse date — dating from conception plus 266 days is more accurate than working backwards to an assumed LMP.

Why the date is an estimate

Only about 4% of babies are born on their estimated due date. The distribution around it is wide:

• Roughly half of births occur within a week either side
• About 90% occur between 37 and 42 weeks
• Term is defined as 37 to 42 weeks; before 37 is preterm, after 42 is post-term

A due date is best understood as the midpoint of a likely range rather than a deadline. Framing it as a single date causes considerable unnecessary anxiety in the final fortnight.

Ultrasound dating

Where LMP dating and ultrasound dating disagree, early ultrasound is more accurate and clinical practice generally revises the due date accordingly.

A first-trimester scan, measuring crown-rump length between roughly 8 and 13 weeks, dates a pregnancy to within about 5 to 7 days. Accuracy declines as pregnancy progresses — a third-trimester scan may be out by three weeks or more, because growth variation between individual babies widens over time.

This is why the dating scan is offered early, and why a due date established in the first trimester is generally not revised later even if growth measurements differ.

Reading the result

The gestational age figure tells you how far along the pregnancy is in completed weeks and days, which is how clinicians describe it — "32 weeks and 4 days" rather than "eight months".

Trimesters are conventionally: first to 13 weeks, second 14 to 27, third 28 to birth.

This is an estimate for general information and is not medical advice. A due date should be confirmed by a doctor or midwife, and any pregnancy should be under proper antenatal care. Irregular cycles, uncertain LMP dates, recent hormonal contraception, and multiple pregnancies all affect dating in ways a formula cannot account for.

Common mistakes

Counting from conception. The convention counts from LMP, two weeks earlier.

Using Naegele's rule with an irregular cycle. It assumes 28 days.

Treating the date as fixed. Only about 4% of babies arrive on it.

Preferring a late scan over an early one. Early dating is considerably more accurate.

Dating from a bleed that was not a true period. Implantation bleeding is sometimes mistaken for one, shifting the estimate by weeks.