Pregnancy Due Date Calculator
Runs locallyInput stays in your browserFree, no sign-up
A due date is computed with Naegele's rule, the formula obstetrics has used for two centuries: take the first day of your last menstrual period (LMP) and add 280 days — 40 weeks. That date is the estimated due date (EDD). One thing surprises almost everyone: pregnancy weeks are counted from the LMP, not from conception. Ovulation usually happens about two weeks after the LMP, so at '8 weeks pregnant' the embryo has only been developing for roughly 6 weeks — that is the medical definition, not an error. If your average cycle is not 28 days, the date shifts by the difference: a 30-day cycle pushes the due date 2 days later, a 26-day cycle pulls it 2 days earlier. This calculator applies exactly that logic — EDD = LMP + 280 days + (average cycle − 28 days) — and derives the current week from the days elapsed since the LMP, with the first/second/third trimester split at 13 and 28 weeks.
Three scenarios cover most visits. First, the positive test: the two lines just appeared and the urgent questions are "how many weeks am I" and "when is the baby due". Enter the LMP and both answers appear instantly, along with a live countdown and which week the first prenatal appointment should fall in. Second, already in care: compare the week on your ultrasound report against the calculator, then scroll to the checkup windows — the NT scan, anatomy scan, glucose screening and the start of fetal monitoring each land on concrete dates, so booking stops being guesswork. Third, partners and family: with the days-to-go number in hand you can plan parental leave and pack the hospital bag on time, and the Copy result button puts the due date and current week into a message for the family chat in one tap.
A few edge cases deserve honesty. First, an unknown LMP or a chaotic cycle: after breastfeeding, stopping birth control, or with PCOS, ovulation drifts and LMP-based math can be off by one to two weeks. The gold standard then is a first-trimester ultrasound — the crown-rump length measured at 8–12 weeks dates the pregnancy more accurately than any formula, and your doctor will revise the due date from it. Second, the due date is not the delivery day: only about 5% of babies arrive on it, and anything between 37 and 42 weeks counts as full term — think of the EDD as the "40 weeks complete" coordinate, not an expiry date. Third, going past 42 weeks is post-term, where complication risks rise; this tool flags it explicitly and you should see your doctor promptly rather than wait at home. Fourth, IVF pregnancies do not use this math: fresh transfers count from the retrieval date, frozen transfers from the transfer date minus 17–19 days — follow your fertility clinic. Fifth, twins and multiples run on a different checkup rhythm and delivery timeline, so treat the windows below as a rough reference.
Using the page is straightforward: pick the first day of your last period up top and enter your average cycle length (28 by default, 21–35 accepted). The due date, current week in "X weeks, Y days" form, trimester, days to go and a pregnancy progress bar update live as you type. Below, five key checkup windows — prenatal filing, NT scan, anatomy scan, glucose screening and fetal monitoring — are listed with their date ranges and a Passed / Now / Upcoming status each. Hit Load example to see a sample calculation first, Copy result to share the key numbers, Clear to start over. Everything is computed locally in your browser — no data leaves your device.
How to use
- Pick the first day of your last period (the day bleeding started, not the day it ended) and enter your average cycle length — results update live, no button needed.
- Read off the due date, current pregnancy week (X weeks, Y days), trimester, days to go and the progress bar.
- Check the five checkup windows below — prenatal filing, NT scan, anatomy scan, glucose screening, fetal monitoring — each marked Passed, Now or Upcoming.
- Copy result sends the due date and week to family for planning; Clear resets the form.
FAQ
How is the due date calculated?
With Naegele's rule: first day of the last period plus 280 days (40 weeks), shifted by the cycle difference when the average cycle is not 28 days — a 30-day cycle adds 2 days. The key subtlety is the starting point: weeks are counted from the last period, not from intercourse or conception, which is why '8 weeks pregnant' means about 6 weeks of embryonic development. That is the medical convention, not a miscalculation.
My ultrasound says a different week than the LMP math. Which is right?
Within a week, both are normal — ovulation does not always happen on day 14. Beyond a week of difference, trust the first-trimester ultrasound: the crown-rump length at 8–12 weeks is the most accurate dating available, and clinicians revise the due date from it. If your cycles are irregular or the LMP is uncertain, treat this calculator as an estimate and go by the early scan.
Will I definitely deliver on my due date? What if I go past it?
Almost certainly not on the day — only about 5% of babies arrive exactly on the EDD, and 37 to 42 weeks all count as full term. Going 1–2 weeks past the due date is common; your provider will monitor with non-stress tests and ultrasounds to decide between waiting and induction. Past 42 weeks is post-term with clearly higher risks — see your doctor promptly instead of waiting at home.
My cycles are irregular. Is this calculator still useful?
Only as a rough estimate. Entering your average cycle over the last six months corrects part of the error, but if the cycle swings by more than ±7 days, ovulation cannot be inferred and the result may be two weeks off. In that case rely on the 8–12 week ultrasound, or look at the trend across several scans — that beats any formula.
Is the date I enter uploaded anywhere?
No. Every calculation on this page runs locally in your browser — the date never touches a server, is never logged, and vanishes when you close the page. It works offline too.