Gestational Age: How to Count Pregnancy Weeks and Work Out the Due Date

September 29, 2026

"How many weeks am I?" is one of the first questions of a pregnancy. The answer follows a convention that confuses many people: gestational age starts counting before conception. Here is how the count works, how to get the due date and the main prenatal milestones.

Pregnancy counts from the last period

Gestational age is counted from the first day of the last menstrual period (LMP), not from the day of conception. The reason is practical: almost everyone knows when their period started, and almost no one knows the exact day of fertilisation. Since ovulation usually happens about two weeks after the LMP, on the day of conception the pregnancy is already "2 weeks" along.

By this convention, a pregnancy lasts on average 40 weeks (280 days).

Due date: Naegele's rule

Due date = LMP + 280 days. A mental shortcut: add 7 days to the LMP, go back 3 months and add 1 year. Example: LMP on 1 January 2026 → 8 January → 8 October 2025 → 8 October 2026.

The rule assumes a 28-day cycle. With longer or shorter cycles ovulation happens later or earlier, and the date moves by the difference: with a 32-day cycle, add 4 days.

When the ultrasound takes over

A first-trimester ultrasound (up to about 13 weeks), which measures the embryo, is the most accurate way to date a pregnancy. When its date differs a lot from the LMP – or the LMP is uncertain – doctors follow the ultrasound. To use that dating, start from the date of the scan and the weeks and days it measured.

Trimesters

Trimester	Weeks
1st	Up to 13 weeks and 6 days
2nd	14 to 27 weeks and 6 days
3rd	28 weeks until birth

Common prenatal milestones

When	What
11 to 14 weeks	First-trimester scan (nuchal translucency)
20 to 24 weeks	Second-trimester anatomy scan
24 to 28 weeks	Glucose tolerance test
37 to 38 weeks and 6 days	Early term
39 to 40 weeks and 6 days	Full term
41 to 41 weeks and 6 days	Late term
42 weeks or more	Post-term

The exact schedule of appointments and tests is set by the team caring for the pregnancy.

Will the baby arrive on the due date?

Rarely on the exact day. Most births happen between 37 and 42 weeks. The due date is a reference for planning prenatal care, not an appointment.

Work out yours

The gestational age calculator counts from the LMP (with your cycle length), an ultrasound or the conception date, and shows the weeks and days, the trimester, the due date and the date of each prenatal milestone. Nothing you type leaves your device.

For information only; it does not replace medical advice. The dating set by the professional caring for your pregnancy is the one that counts.

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