Pregnancy Due Date Calculator
From your last period or conception date — your estimated due date and the trimester map of the next nine months.
Estimate your due date
How due dates are calculated
The standard method is Naegele's rule, in use since the 1800s: due date = first day of your last menstrual period plus 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14 — which is why counting from a known conception date instead uses 266 days. Pregnancy is dated from the LMP, which produces the famous oddity that you're "two weeks pregnant" before conception occurs.
What the date actually means
A due date is the center of a probability curve, not an appointment. Only about 4–5% of babies arrive on their due date; the great majority arrive within two weeks either side, and "full term" formally spans 39 weeks 0 days to 40 weeks 6 days. First pregnancies tend to run slightly past the date. Your provider will refine the estimate with a first-trimester ultrasound, which measures the embryo directly and is accurate to within about 5–7 days — when ultrasound and LMP disagree meaningfully, the early ultrasound wins.
The trimester map
First trimester (weeks 1–13): organ formation, the most fatigue, the first prenatal visits. Second (14–27): typically the most comfortable stretch — the anatomy scan lands around week 20. Third (28–40): growth and the home stretch, with more frequent checkups. This calculator marks each boundary so you can plan the practical things — announcements, travel cutoffs, leave paperwork — around real dates. And the obvious but important note: this tool estimates; your OB or midwife dates the pregnancy officially.
Frequently asked questions
My cycles aren't 28 days — is the date still right?
Naegele's rule assumes ovulation on day 14. Longer cycles usually mean later ovulation and a later true due date (roughly: add the days your cycle exceeds 28). The first-trimester ultrasound resolves this properly.
Which is more accurate, LMP or ultrasound?
A first-trimester ultrasound, measured before week 14, is the most accurate dating available (±5–7 days). LMP dating works well with regular cycles and a confidently-remembered date; many providers use both.
When should I tell people?
Entirely personal. Many wait until after the first trimester (around week 13) when miscarriage risk drops substantially, but there is no rule — tell whoever you'd want support from, whenever you wish.
When does the baby actually come?
Statistically: a broad bell curve centered near 40 weeks, with first babies skewing a few days later. About 60% arrive by the due date, and the large majority within 10 days after. Pack the bag by week 36.
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Why your due date is an estimate, not an appointment
A due date is calculated by adding two hundred and eighty days - forty weeks - to the first day of your last menstrual period. This is Naegele's rule, it dates from the early nineteenth century, and it assumes a twenty-eight day cycle with ovulation on day fourteen. Plenty of people do not have a twenty-eight day cycle, which is the first reason the number moves.
The second reason is simply biology. Only a small minority of babies arrive on their estimated date. The great majority arrive within a couple of weeks either side of it, which means the honest way to read your due date is as the center of a window rather than a deadline. Knowing that in advance saves a great deal of anxiety in week forty-one.
What the weeks are counting
Pregnancy is dated from the last period rather than from conception, which means the first two weeks of a "pregnancy" happen before there is one. It is a convention, not a mistake: the date of the last period is something most people can report accurately, and the date of conception usually is not. If you know your conception date, it is generally about two weeks later than the date this calculation starts from.
The trimesters divide roughly into weeks one to thirteen, fourteen to twenty-seven, and twenty-eight onwards. Full term is considered thirty-nine to forty weeks and six days. Thirty-seven to thirty-eight weeks is early term, and forty-one weeks onwards is late term. These labels matter mainly because they are the vocabulary your midwife or doctor will use.
Please treat this as arithmetic, not advice
This calculator does date arithmetic. That is all it does, and it is all it should be trusted for. An early ultrasound measurement is a more accurate way to date a pregnancy than any formula, because it measures the actual pregnancy rather than assuming an average cycle - and if a scan gives you a different date to this page, the scan is right.
Nobody involved in building this site is a doctor, a midwife, or any kind of medical professional, and this page is not medical guidance. Every question about your pregnancy belongs with the people caring for you. We have tried to make the arithmetic clear and honest; the rest is not ours to offer.