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Pregnancy Due Date Calculator

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Naegele's rule assumes 28. 20–45 accepted.

Estimated due date

Monday, December 7, 2026

About 4% of babies are born on this date. It is the middle of a five-week window, not an appointment.

  • Gestational age on that date27w 0d
  • TrimesterSecond trimester
  • Days to go91
  • Conception, if ovulation was typicalMar 16, 2026
How this was worked out
  • Dated fromThe first day of the last period, Mar 2, 2026
  • Adjusted for cycle length28-day cycle: +0 days against the 28-day assumption Naegele's rule makes
  • Due dateMonday, December 7, 2026 — 280 days from the last period
Weeks along27w 0d
Birth today would bePreterm
Weeks in total40

Dates along the way

  • End of first trimester (13w 6d)Jun 7, 2026
  • End of second trimester (27w 6d)Sep 13, 2026
  • Early term begins (37w)Nov 16, 2026
  • Full term begins (39w)Nov 30, 2026
  • Postterm begins (42w)Dec 21, 2026

This estimates a due date. It does not date a pregnancy, and it is not medical advice. Dating is done at a scan: ACOG holds that a first-trimester ultrasound measurement is the most accurate way to establish gestational age, good to within about 5–7 days, and a clinic will re-date from it when it disagrees with the calendar. If you have a scan date, that one is the one to use.

Naegele’s rule adds 280 days — 40 weeks — to the first day of the last period. It assumes a 28-day cycle with ovulation on day 14, so the cycle length above shifts the date by the difference, which is Parikh’s adjustment. Gestational age is counted from that first day rather than from conception, which is why a pregnancy is described as four weeks along about two weeks after it began. Term is a five-week window: early term at 37 weeks, full term at 39, postterm at 42.

A due date is the middle of a window

Only about four babies in a hundred arrive on their estimated date. That is not a failure of the estimate — it is what the estimate means. Birth timing is spread across roughly five weeks, with about two thirds arriving within ten days either side. The date is a useful anchor for appointments, screening windows and leave paperwork, and a misleading prediction of any particular morning.

This calculator gives that anchor from whichever date you actually know, and then shows how far along a pregnancy is on any date you choose, which trimester that falls in, and when each of the term thresholds is reached.

A worked example from 2 March 2026

With a first day of the last period on 2 March 2026 and a typical 28-day cycle, the estimated date is 7 December 2026. On 16 August that pregnancy is 23 weeks and 6 days along, in the second trimester, with 113 days to go.

Change the cycle to 32 days and the date moves to 11 December, because ovulation happens later in a longer cycle and the count really runs from ovulation. You may also see 9 December quoted for the same period, from the calendar-month form of the rule — add nine months, then seven days. That version drifts a day or two depending on which months a pregnancy spans; this tool adds a flat 280 days, which is what dating wheels and clinical software use.

Three dates you might have instead

If you know when you conceived or ovulated, the count is 266 days from there rather than 280 from the period, the difference being the two weeks the rule assumes between the two.

After IVF the date is known precisely, so nothing is assumed at all. An embryo transferred on day five is already five days past fertilisation, so the count is 261 days from the transfer; a day-three transfer counts 263. In the worked example above, a day-five transfer on 21 March 2026 gives the same 7 December.

Why a scan outranks any calendar

Naegele’s rule dates from 1812 and assumes a 28-day cycle with ovulation on day 14. Real cycles vary, and ovulation varies more than cycle length does, so the calendar carries real uncertainty even when the dates are remembered correctly. ACOG holds that measuring the embryo in the first trimester is the most accurate way to establish gestational age, good to within five to seven days, and clinics re-date from the scan when the two disagree.

One more convention causes constant confusion: gestational age is counted from the first day of the last period, not from conception. That is why a pregnancy is described as four weeks along about two weeks after it started, and why a test taken the day a period is missed already reads as four weeks.

Questions about estimated due dates

What counts as full term?

ACOG replaced the single five-week label in 2013 with four stages, because outcomes differ measurably across it. Early term runs from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks to 40 weeks 6 days, late term through 41 weeks 6 days, and postterm from 42 weeks.

My cycles are irregular. Is this any use?

Less than it would otherwise be. The adjustment for cycle length assumes ovulation lands a fixed fourteen days before the next period, which holds up poorly when cycles vary a lot. Take the result as a rough placeholder and get a first-trimester scan, which does not depend on remembering or predicting anything.

Can the date be changed later?

Yes, and it often is. If an early scan disagrees with the calendar estimate by more than a threshold that depends on how far along the pregnancy is, the scan date replaces it and becomes the official one. Later scans date much less reliably, because babies of the same age vary more in size as they grow.

Why does the count start two weeks before conception?

Because the first day of a period is a date people can identify and the moment of conception usually is not. Obstetrics standardised on the observable event, so the first two weeks of a forty-week pregnancy happen before there is anything to be pregnant with. Every clinic, chart and scan uses the same convention.

Sources

Last reviewed 27 August 2026