Pregnancy Due Date Calculator

Three ways in, one timeline

All three starting points are converted to the same clock — days since the first day of the last period, counted the way clinicians count a pregnancy — so switching between them does not move the timeline, it only changes how much you have to guess.

Naegele's rule (LMP + 280 days) assumes a 28-day cycle with ovulation on day 14. If your cycles run 32 days, the estimate moves four days later — 23 December instead of 19 December on the default inputs — because ovulation happened later than the rule assumes. That is the single most common reason a calendar due date and an ultrasound due date disagree.

What "weeks pregnant" actually counts

Gestational age is counted from the last period, not from conception, which is why the first two weeks of a pregnancy happen before it begins. On the default dates:

31 weeks 6 days = 223 days since 14 March 2026
Third trimester  started 26 September 2026 (28 weeks)
Full term        from 12 December 2026 (39 weeks)
Due date         19 December 2026 (40 weeks)

Those thresholds are not trivia — they drive real decisions. 37 weeks is early term, 39 weeks is full term, and 24 weeks is the threshold of viability that obstetric units work from. The milestones list in the calculator dates each of them for you rather than making you count backwards from 40.

Which date should you trust?

• A first-trimester ultrasound beats any calendar. Between roughly 8 and 13 weeks, crown-rump length dates a pregnancy to within about five days, and clinicians will move the due date if the two disagree by more than that.
• IVF dates are the most reliable calendar method. There is no cycle-length guesswork when the transfer day is known.
• LMP is the weakest of the three — irregular cycles, breastfeeding, recent hormonal contraception, and simple misremembering all degrade it.
• Later ultrasounds date less accurately. By the third trimester, growth variation between babies is larger than the dating error, so a due date is rarely changed on a late scan.

About the due date itself

The due date is the middle of a range, not an appointment. Only about one baby in twenty arrives on that exact day; about half arrive within a week of it, and the normal window runs from 37 weeks to 42 weeks. A "post-term" label starts at 42 weeks, not at 40 weeks and one day.

How to use this calculator

1. Choose what you are dating from: last period, conception, or an IVF transfer day.
2. Enter that date.
3. If you are using the last period, enter your usual cycle length.
4. Set the date you want the gestational age for — today, or your next appointment.
5. Read the due date, then the gestational age and the milestone dates underneath it.

Frequently asked questions

How accurate is a due date calculator?

As a calendar estimate, it is accurate to about a week at best. A first-trimester ultrasound is more accurate still, dating a pregnancy within roughly five days, and that measurement overrides the calendar when the two disagree.

Why does my cycle length change the date?

Because ovulation — not the period — is the biological starting point. A 32-day cycle ovulates around day 18 rather than day 14, so the due date moves four days later than the standard 280-day rule would put it.

Can I use a conception date instead?

Yes. Add 266 days. If your conception date and last period disagree by more than a few days, the conception date is usually the better one, because it does not depend on an assumption about cycle length.

How is the due date calculated after IVF?

From the transfer date: 261 days for a day-5 blastocyst, 263 days for a day-3 embryo. It is the most reliable of the three methods because the transfer date is known exactly.

What counts as full term?

39 weeks is full term and 37 weeks is early term. Babies born between 37 and 39 weeks are generally healthy, but elective deliveries are usually held until 39 weeks unless there is a medical reason.

Does the due date change after an ultrasound?

Sometimes. If a first-trimester scan differs from the calendar estimate by more than about five days, most clinicians move the due date to the scan. Later scans rarely change it.

Data sources: Due dates use Naegele's rule (first day of the last period plus 280 days, adjusted by the difference between your cycle length and 28 days), 266 days from a known conception date, and 261 or 263 days from a day-5 or day-3 IVF transfer — the same offsets used in standard obstetric dating. Gestational age is counted from the LMP-equivalent date. Trimester boundaries are 14 and 28 weeks; early term is 37 weeks and full term 39 weeks. Dates are computed in UTC. This is an estimate for information only and is not medical advice: only a clinician dating your pregnancy, usually by ultrasound, can set a due date. Every figure comes from the calculator above and nothing you enter is stored. For other health numbers, see the BMI calculator and the calorie calculator.