Pregnancy due date calculator
An estimated due date from your last period, a known conception date, or an IVF transfer, along with your current gestational week and the dates key milestones fall on.
Built by Mubashir. Uses a published standard formula, named below. Not medical advice — see the limitations section.
Your dates
Standard dating assumes 28 days. Longer or shorter cycles shift the estimate.
Estimated due date
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Currently
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Trimester
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Days remaining
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Estimated conception
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| Milestone | Gestational week | Date |
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Show the working
—Follow your midwife or doctor, not this calculator. A dating ultrasound is considerably more accurate than any date-based estimate, particularly in the first trimester, and your care team's date supersedes this one. Only about four percent of babies arrive on their estimated due date. This tool cannot assess anything about your pregnancy and is not a substitute for antenatal care.
Why pregnancy is dated from your last period
This confuses almost everyone at first. Gestational age is counted from the first day of your last menstrual period, which is roughly two weeks before conception actually occurred. By the convention, you are considered two weeks pregnant at the moment of conception.
The reason is practical rather than biological. The date of the last period is something most people can report; the date of conception usually is not. Clinical dating standardised on the knowable figure.
This is why a forty-week pregnancy involves about thirty-eight weeks of actual gestation, and why the conception date shown above sits two weeks after your last period rather than at the start.
Naegele's rule and its assumptions
The standard calculation adds 280 days to the first day of the last period. It dates to the early nineteenth century and assumes a regular 28-day cycle with ovulation on day fourteen.
That assumption does not hold for many people. Cycles commonly run anywhere from 21 to 35 days, and ovulation timing varies within them. The calculator adjusts for cycle length, adding or subtracting the difference from 28 — so a 32-day cycle pushes the estimate four days later. If your cycles are irregular, the adjustment is guesswork and an early ultrasound will be substantially more reliable.
IVF dating is the exception: because the transfer date is known precisely, the estimate is far more accurate than any last-period calculation.
How much a due date actually tells you
Less than the precision of a specific date implies. Roughly four percent of babies are born on their estimated due date. Term is defined as a five-week window from 37 to 42 weeks, and the great majority of births fall somewhere in it.
First pregnancies tend to run slightly longer than subsequent ones. The estimated date is better understood as the centre of a distribution than as a deadline, and it is worth mentally preparing for a range of a couple of weeks either side.
An early ultrasound, particularly between weeks eight and thirteen, dates a pregnancy within a few days and will usually override a last-period estimate if the two disagree by more than about a week.
The milestone dates
The table shows when key points fall. The end of the first trimester at thirteen weeks marks a substantial drop in miscarriage risk. The anatomy scan, typically offered between eighteen and twenty-two weeks, checks fetal development in detail. Thirty-seven weeks is the threshold for full term. Forty weeks is the estimate itself.
These are general reference points. Antenatal schedules differ by country and by individual circumstances, and your care team will give you the appointments that actually apply to you.
Frequently asked questions
How accurate is a due date?
About four percent of babies are born on the estimated date. Term spans 37 to 42 weeks, and most births fall within that five-week window. Treat the date as the centre of a range.
Why is pregnancy dated from my last period?
Because that date is usually knowable while the date of conception is not. It means gestational age runs about two weeks ahead of actual embryonic age, so you are counted as two weeks pregnant at conception.
What if my cycle is irregular?
The estimate becomes unreliable, because the calculation assumes a predictable ovulation date. An early dating ultrasound is considerably more accurate and is what your care team will rely on.
What counts as full term?
Thirty-seven weeks and beyond is generally considered full term, though some guidance distinguishes early term at 37 to 38 weeks from full term at 39 to 40. Before 37 weeks is preterm.
Is IVF dating more accurate?
Yes, substantially. Because the transfer date is known exactly, the estimate does not depend on assumptions about cycle length or ovulation timing.
Should I trust this over my ultrasound date?
No. A first-trimester ultrasound dates a pregnancy within a few days and takes precedence over any calculation from dates. Always follow the date your care team gives you.