How is a pregnancy due date calculated, and why it's an estimate

A pregnancy due date is an estimate of the day the pregnancy reaches 40 weeks. The most common method, Naegele's rule, counts 280 days from the first day of your last menstrual period (LMP). Most people deliver within a few weeks either side of that date, so treat it as the centre of a window, not a fixed appointment. A clinician may change it after an early ultrasound.

Pregnancy due date calculatorWhen is my baby due?Do it free

Why the count starts from the last period

Pregnancy is counted from the LMP rather than from conception, because the first day of the last period is easy to recall and the exact day of ovulation usually is not known. Conception typically happens about two weeks after the LMP in a 28-day cycle. Counted from conception, the pregnancy lasts about 266 days, or 38 weeks. The two counts describe the same pregnancy, which is why you may hear both "38 weeks" and "40 weeks".

Naegele's rule, step by step

1. Take the first day of your last period.
2. Add 280 days (40 weeks).
3. Check the result against a calendar, or use a date calculator to count the days.

For example, an LMP of 1 March 2026 gives a due date of 6 December 2026. The rule is named after Franz Karl Naegele, a German obstetrician of the 19th century.

Cycle length changes the date

The 280-day count assumes ovulation around day 14 of a 28-day cycle. If your cycle is longer, ovulation and conception usually come later, so the due date moves later too. A common adjustment is to add the difference between your cycle length and 28 days. A 35-day cycle adds 7 days, so the 1 March 2026 example becomes 13 December 2026.

Many calculators apply this adjustment, but it is not applied everywhere. ACOG's committee opinion on estimating the due date lists irregular cycles as one reason LMP dating is imprecise, which is why clinicians often confirm the date with a scan. Ask your clinician which method they use.

Conception and IVF dates

If you know the date of conception, add 266 days. After in-vitro fertilisation (IVF), clinics count from the day the embryo was transferred, adjusted for the embryo's age. The arithmetic works out as transfer date plus 266 days minus the embryo's age in days. For a day-3 embryo, that is transfer date plus 263 days. For a day-5 blastocyst, it is transfer date plus 261 days.

For a transfer on 1 March 2026, a day-3 embryo gives 19 November 2026 and a day-5 embryo gives 17 November 2026. ACOG states that for pregnancies conceived with assisted reproduction, the due date should be set from the age of the embryo and the transfer date. Use the date your clinic gives you, because clinics may use their own conventions.

Dating by ultrasound

ACOG's committee opinion on estimating the due date (Committee Opinion No. 700, 2017) says that an ultrasound in the first trimester, up to and including 13 6/7 weeks, is the most accurate way to establish or confirm gestational age. The margin for crown-rump length measurements is about 5 to 7 days. Clinics may move the due date when a scan and the LMP disagree, and the threshold depends on how far along you are. Ask your clinician to explain any change.

Weeks and days

Pregnancy dates are written as weeks plus days, such as 24w3d (24 weeks and 3 days). With an LMP of 1 March 2026, the pregnancy is 253 days old on 9 November 2026, which is 36 weeks and 1 day. Counting the days between any two dates works the same way; see days between dates.

Trimesters and term stages

Two sets of definitions are common, and they differ slightly.

ACOG defines the trimesters as follows:

• First trimester: up to 13 6/7 weeks
• Second trimester: 14 0/7 to 27 6/7 weeks
• Third trimester: 28 0/7 weeks onward

ACOG's Committee Opinion No. 579 (2013) replaced the single word "term" with four categories for deliveries at or after 37 weeks:

• Early term: 37 0/7 to 38 6/7 weeks
• Full term: 39 0/7 to 40 6/7 weeks
• Late term: 41 0/7 to 41 6/7 weeks
• Postterm: 42 0/7 weeks and beyond

ACOG's patient-facing pages sometimes use slightly different cut-offs, so follow the wording your own clinician uses. Some coding systems also use different trimester boundaries.

Quick answers

Is my due date a fixed day? No. It is a best estimate, and most babies arrive within a window around it.

Why did my due date change at the scan? Early ultrasound measures the baby's size more accurately than a remembered period date, especially when cycles are irregular.

Can I calculate from my conception date? Yes, add 266 days, or use the IVF transfer rule if you had embryo transfer.

Does the calculator replace my midwife's dating? No. It is a way to understand the arithmetic. Your clinician's dating is the one to use.

This is general information, not medical advice. Contact your midwife, obstetrician or GP to confirm your due date. Get medical help promptly for bleeding, severe pain, or anything that worries you.