Pregnancy Due Date Calculator

Choose how to date the pregnancy — the first day of the last menstrual period, the conception date, or the date of an IVF embryo transfer — and the calculator returns the estimated due date, the gestational age on any date you pick, the trimester that date falls in, and the days remaining until the due date. The due date is the headline, but gestational age is the number clinicians work in: pregnancy is tracked in completed weeks and days, written as 12 weeks, 3 days, never in months. The as-of date defaults to today.

How each method sets the date

All three methods aim at the same target, 280 days of gestational age, and differ only in which known date anchors the count. The American College of Obstetricians and Gynecologists states the convention in Committee Opinion 700: the due date is 280 days after the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14.

due date = last period + 280 days + (cycle − 28) days

The 280 days are exactly 40 weeks. For cycles longer or shorter than 28 days the calculator shifts the estimate by one day per day of difference, since ovulation falls later in longer cycles. That adjustment is standard calculator practice built on ACOG's stated assumption, not an ACOG formula; clinics handle irregular cycles with an ultrasound. Dating from conception drops the two weeks the menstrual count spends before ovulation:

due date = conception + 266 days

The 266 is simply 280 minus the assumed 14 days between the period and ovulation. IVF is the one situation where guesswork disappears, because the embryo's age on the day of transfer is known exactly:

due date = transfer + (266 − embryo age) days

The 261 days for a day-5 embryo and 263 for a day-3 embryo are ACOG's own figures, and the opinion directs that a pregnancy from assisted reproduction be dated this way rather than from menstrual history. Whatever the method, the calculator then counts back 280 days from the due date to fix the start of the gestational clock, from which every progress figure is measured.

A worked example

Take a last period beginning on 1 January 2026, day 1 of the year, and a 28-day cycle: 1 + 280 = 281, so the due date is the 281st day of 2026. The month lengths from January through September — 31, 28, 31, 30, 31, 30, 31, 31, 30 — use up 273 days, leaving 8, so day 281 is 8 October and the calculator reports Thursday, 8 October 2026. Progress as of 23 August 2026, day 235 of the year, puts the gestational age at 235 − 1 = 234 days, or 33 weeks, 3 days, past the 28-week line and so in the third trimester, with 281 − 235 = 46 days to go. Change the cycle length to 30 and everything slides two days: the due date becomes Saturday, 10 October 2026, and the gestational age on the same August date drops to 33 weeks and 1 day, because the assumed start of the pregnancy moved from 1 January to 3 January.

Gestational age is not embryonic age

The gestational clock starts on the first day of the last period, roughly two weeks before conception, so at the moment of conception a pregnancy is already about 2 weeks along by the medical count, and someone described as 6 weeks pregnant carries an embryo roughly four weeks old. Embryonic age, counted from fertilization, runs a constant 14 days behind — the same offset that turns 280 days from the period into 266 from conception. Medicine settled on the menstrual count because the first day of a period is a date many people can name, while ovulation has to be inferred; even so, Committee Opinion 700 reports that only about half of women accurately recall their last menstrual period. Every output here uses the gestational count, whichever anchor you supplied.

Where the 280-day count comes from

The rule is named for Franz Carl Naegele (1778–1851), professor of obstetrics at Heidelberg from 1810, who set out the calculation in an 1812 textbook without claiming to have invented it. He quoted Hermann Boerhaave (1668–1738), professor of botany and medicine at Leyden, who never published it either: Boerhaave discouraged printed records of his lectures, and it was Albrecht von Haller, who attended them around 1727, who published the Academic Lectures in 1744, six years after Boerhaave's death. The relevant passage, in the translation by Baskett and Nagele that Loytved and Fleming reproduce, holds that "of one hundred births altogether, ninety-nine came about in the ninth month after the last menstruation by counting one week after the last period and by reckoning the nine months of gestation from that time." It is a lecturer's remark, not a described study.

Naegele's midwifery textbook, first published in Heidelberg in 1830, ran through eight editions by 1850 on Loytved and Fleming's count, though other tallies run higher, and from the second edition of 1833 it carried a worked example: last period on 10 June, count back three months to 10 March, add seven days, due on 17 March. Which day of the period the count started from stayed ambiguous in his wording until the seventh edition of 1847 fixed it to the first day. In 2000 Baskett and Nagele argued in BJOG that the rule may have been misinterpreted, that Naegele might have meant the last day of the period, which would push every due date about five days later; the suggestion entered midwifery textbooks, and popular retellings added the flourish that American authors had quietly switched the rule. Loytved and Fleming went back to the original German editions in 2016 and found the opposite: Naegele clearly advised counting from the first day. The last-day story is a debunked anecdote, though the finer textual details rest on Loytved and Fleming's reading alone. What endures is the round number.

What the due date actually predicts

An estimated due date is the middle of a distribution, not an appointment. Only about 4 percent of women deliver on the 280th day, and only about 70 percent within ten days of it, figures from Mongelli and colleagues in 1996, still quoted by Jukic and colleagues in 2013 when they timed 125 naturally conceived births from ovulation: the median was 268 days, or 38 weeks and 2 days, and even after excluding preterm births the spread ran 37 days. Lawson's 2021 review concludes that the rule should be treated as a guideline, not a definite date, which is why the days-until figure here goes negative once the date has passed. Hugh Robinson showed in 1973 that a crown–rump measurement between the sixth and fourteenth weeks predicts maturity to within three days, and ACOG's Committee Opinion 700, issued in May 2017 with the AIUM and SMFM, makes first-trimester ultrasound the most accurate method. Its thresholds for replacing a menstrual date with the scan date, given here from the opinion's text as reproduced in the Alaska Native Medical Center's dating guideline, are a discrepancy of more than 5 days up to 8 weeks 6 days, more than 7 days from 9 weeks through 15 weeks 6 days, more than 10 days through 21 weeks 6 days, more than 14 days through 27 weeks 6 days, and more than 21 days from 28 weeks onward, where ultrasound is the least reliable method at plus or minus 21 to 30 days. A pregnancy with no scan confirming or revising the date before 22 weeks is, in the opinion's own phrase, suboptimally dated. A date confirmed at a scan therefore outranks anything computed from remembered dates.

This tool performs date arithmetic and nothing else. It examines no one, and its output is not medical advice: pregnancy dating is established and confirmed by a clinician, usually with a first-trimester ultrasound, and decisions about care belong in that conversation. Treat every figure here as provisional until then, and see the site disclaimer.

Assumptions and limitations

Trimester labels follow the common week-based convention: first while gestational age is under 14 weeks, second from 14 weeks, 0 days up to but not including 28 weeks, third from 28 weeks, 0 days onward; other schemes split at 13 or 27 weeks and are not wrong. Gestational ages beyond exactly 42 weeks are flagged in the output, since a date that far past term is more likely a data-entry slip. Cycle length is accepted between 20 and 45 days, and because a long cycle shifts the assumed start of the pregnancy later than the entered period date, an as-of date earlier than that shifted start is rejected with an error rather than shown as negative progress. All dates are anchored at midday UTC before subtraction, so time zones and daylight-saving changes never nudge a result across a day boundary.

Frequently asked questions

How do you calculate a due date from your last period?

Count 280 days, which is 40 weeks, from the first day of the last menstrual period. This calculator also shifts the result by one day for each day the average cycle differs from 28, so a 31-day cycle adds three days. The count is known as Naegele's rule after the Heidelberg obstetrician who set it out in an 1812 textbook, though Naegele himself credited Hermann Boerhaave, whose Leyden lectures, published posthumously in 1744, already contain the calculation.

How many weeks pregnant am I?

Gestational age is the time since the assumed start of the pregnancy, counted in completed weeks plus leftover days. Under the standard convention the start is the first day of the last period, so someone whose period began exactly ten weeks before today is 10 weeks, 0 days pregnant, even though conception came roughly two weeks after that start.

How is an IVF due date calculated?

From the embryo transfer date. Because the embryo's age is known exactly, a day-5 transfer adds 261 days and a day-3 transfer adds 263 days, so both land on the same target of 266 days from fertilization that conception dating uses.

What is the difference between gestational age and embryonic age?

Gestational age counts from the first day of the last menstrual period, while embryonic age counts from fertilization, which arrives about two weeks later. The two clocks run a constant 14 days apart under the standard 28-day-cycle assumption, which is why 280 days from the period equals 266 days from conception.

How likely is a baby to be born exactly on the due date?

Rarely. Only about 4 percent of women deliver on the 280th day, and only about 70 percent within ten days of it, figures from a 1996 study by Mongelli and colleagues that are still quoted in the research literature. The due date marks the middle of a spread of likely birth dates rather than a prediction of a particular day, and clinicians use it as a reference point for tracking growth and timing care, not as a forecast.

Can a due date change after an ultrasound?

Yes. ACOG's Committee Opinion 700 treats ultrasound in the first trimester as the most accurate way to establish gestational age and gives thresholds for replacing a menstrual due date with the scan date: a disagreement of more than 5 days up to 8 weeks 6 days, more than 7 days from 9 weeks through 15 weeks 6 days, more than 10 days through 21 weeks 6 days, more than 14 days through 27 weeks 6 days, and more than 21 days from 28 weeks onward. A date confirmed by a clinician supersedes anything computed from calendar arithmetic.