About this calculator
A due date is an estimate built on a tidy fiction: that conception happened exactly 14 days after the first day of your last menstrual period (LMP), and that pregnancy lasts exactly 280 days from that point. Real cycles vary, so providers treat the LMP due date as the starting estimate and refine it with an early ultrasound, which measures the embryo directly.
Statistically, only about 5% of babies are born on their due date. A normal full-term delivery can happen any time from 37 to 42 weeks — a five-week window. The due date is best understood as the middle of that window, useful for scheduling check-ups and planning leave, not as an appointment.
How to use it
- Enter the first day of your last menstrual period (not the last day, and not the day you think you conceived).
- Read your due date (LMP + 280 days), your current week of pregnancy, and your trimester.
- If your cycles are usually much longer or shorter than 28 days, mention it to your provider — they will adjust, and our Ovulation Calculator shows how cycle length shifts the dates.
The formula
Pregnancy is conventionally dated from the LMP because it is a date most people know, while conception is not. The count includes roughly two weeks before conception even occurred — which is why a "four-week" pregnancy test result is dated from before the egg was fertilized.
Worked example
If your last period started on Jan 15, 2026, your due date is Oct 22, 2026 (Jan 15 + 280 days), with estimated conception around Jan 29. On Apr 9, 2026 you would be exactly 12 weeks pregnant — the end of the first trimester — with 196 days to go.
Frequently asked questions
How accurate is a due date?
As a prediction of the exact day, not very: only about 1 in 20 babies arrive on it. As a planning anchor it is solid — roughly 70% of babies are born within 10 days of the due date. First-trimester ultrasound dating is more accurate than LMP dating when the two disagree, because early embryo size varies very little.
My cycle is longer than 28 days. Does that change my due date?
Yes, slightly. The 280-day rule assumes ovulation on day 14; with a 35-day cycle, ovulation is closer to day 21, so the true due date is about a week later. Providers adjust for known cycle length or ultrasound measurements — this is one reason early scans sometimes "move" a due date.
What are the trimester boundaries?
The first trimester runs to week 13, the second from week 14 to week 27, and the third from week 28 until delivery. Key milestones: the anatomy scan around weeks 18–22, glucose screening around weeks 24–28, and full term at 39–40 weeks (early term 37–38, late term 41).
When should I see a doctor after a positive test?
Most providers schedule the first prenatal visit around weeks 8–12. Call sooner if you have pain, bleeding, a history of ectopic pregnancy, or chronic conditions or medications that need review. This calculator is an estimate for planning, not medical advice.