Due date calculator: your estimated due date and term window

Estimated due date from a last period of Thu, Jan 1, 2026worked example

Thu, Oct 8, 2026

Falls on a: Thursday
Term window (37-42 weeks): Thu, Sep 17, 2026 to Thu, Oct 22, 2026ACOG and the NHS both call this range term
Estimated conception: Thu, Jan 15, 2026EDD minus 266 days
Second trimester begins: Thu, Apr 9, 2026
Third trimester begins: Thu, Jul 16, 2026

A due date is the midpoint of a distribution, not an appointment. ACOG describes the 280-day rule as a convention that assumes a regular 28-day cycle with ovulation on day 14, and treats a first-trimester ultrasound as the more accurate way to establish gestational age. The five-week term window above is the honest version of the answer.

What date do you have?

First day of your last periodDay 1 is the first day of full bleeding, not spotting.Average cycle length (days)The 280-day convention assumes 28. A longer cycle usually ovulates later, so the estimate shifts with it.

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

What the dates mean
Milestone	Date	Gestational age
First day of last period	Thu, Jan 1, 2026	0w 0d
Second trimester begins	Thu, Apr 9, 2026	14w 0d
Third trimester begins	Thu, Jul 16, 2026	28w 0d
Start of term	Thu, Sep 17, 2026	37w 0d
Estimated due date	Thu, Oct 8, 2026	40w 0d
End of term	Thu, Oct 22, 2026	42w 0d

Trimester boundaries at 13w6d and 27w6d are a widely used convention rather than a finding. Term is 37 to 42 weeks in both ACOG and NHS patient guidance.

The sources this defers to

ACOG Committee Opinion 700: Methods for Estimating the Due Date — the 280-day convention, and why a first-trimester scan beats it
NHS: due date calculator and pregnancy dating — the NHS puts pregnancy at 37 to 42 weeks from the first day of your last period

Working the other way round? The conception date calculator runs the same convention backwards, and the weight-gain ranges are indexed by the gestational week above.

A due date calculator adds 280 days to the first day of your last period, the convention ACOG publishes, and shifts it when your cycle is not 28 days long. It returns the estimated due date, gestational age, trimester dates and the 37 to 42 week window both ACOG and the NHS call term.

Estimated due date from the last menstrual period — EDD = first day of LMP + 280 days. Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (ACOG states this convention assumes a regular 28-day cycle with ovulation on day 14, and does not account for inaccurate recall of the last period, irregular cycle length, or variation in when ovulation actually happened. ACOG treats a first-trimester ultrasound as the most accurate way to establish gestational age, and reports that about half of women recall their last period accurately.)
Cycle-length adjustment to the due dateconvention — EDD = LMP + 280 days + (cycle length - 28). Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (A derived adjustment, not an ACOG recommendation. It assumes the whole difference in cycle length falls in the follicular phase, which is where Bull 2019 found most of the variation, but the luteal phase varies too.)
Gestational age in weeks and daysconvention — gestational age (days) = 280 - days until EDD. Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (Inherits every uncertainty of the due date it is measured from. Trimester boundaries (13w6d and 27w6d) are a widely used convention rather than a finding.)
Estimated conception date from the due dateconvention — conception date = EDD - 266 days. Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (The result is a window, not a day. Sperm can survive several days in the reproductive tract, so intercourse days earlier can produce a conception dated here, and ovulation timing itself varies.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

Frequently asked questions

How is a due date calculated?

By convention the estimated due date is 280 days after the first day of the last menstrual period. ACOG states plainly that this assumes a regular 28-day cycle with ovulation on day 14, and does not account for recall error, irregular cycles or variation in ovulation timing.

What if my cycle is not 28 days?

A longer cycle usually means later ovulation, so the same last period implies a later due date. This tool shifts the estimate by your cycle length minus 28. That adjustment is arithmetic on ACOG's stated assumption rather than an ACOG recommendation, and it is labelled as a convention.

Why did the scan give a different due date?

ACOG treats a first-trimester ultrasound as the most accurate way to establish gestational age, and reports that in one study 40 percent of women had their due date adjusted after a first-trimester scan because it differed from the last-period date by more than five days. The scan wins.

How likely is the baby to arrive on the due date?

The due date is the midpoint of a distribution, not an appointment. ACOG and the NHS both describe 37 to 42 weeks as term, which is a five-week window, and this tool prints those dates alongside the estimate for exactly that reason.

Can I work out a due date from a conception or transfer date?

Yes. Where the conception or ovulation date is known, the due date is 266 days later - 280 minus the 14 days the convention assumes before ovulation. Switch the input above to use it, and the arithmetic is shown in full below the result.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Pregnancy weight gain calculatorThe gestational week above is the input the IOM weight-gain ranges are indexed by.Conception date calculatorThe same convention run backwards, with the window it implies.

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Conception date calculatorEarly pregnancy Pregnancy weight gain calculatorPregnancy health Ovulation calculatorCycle and ovulation