How your due date is calculated
The method behind every due date — including the one your doctor gives you — is older than you might expect. Naegele's rule, named after a German obstetrician working in the early 1800s, counts 280 days (40 weeks) from the first day of your last menstrual period. Not from conception: from the period before it. That quirk exists because most people know when their period started but not when they ovulated, so medicine standardised on the date everyone could actually report.
It also means that during the first two weeks of your official pregnancy, you weren't pregnant yet — the count starts before conception. If you know your conception date instead (from ovulation tracking or IVF), the calculator counts 266 days from there and lands on the same answer by a different road.
The 28-day assumption is the rule's weak spot. If your cycles run longer or shorter, ovulation shifts, and the calculator adjusts for that when you enter your cycle length — a correction the original 19th-century rule never made.
How accurate is a due date, really?
Here's the statistic worth framing: only about one baby in twenty-five arrives on the estimated due date. It's not a deadline — it's the midpoint of a distribution. Full term spans 37 to 42 weeks, most babies arrive within a week or so of the estimate on either side, and first babies run slightly later on average. Prepare for the window, not the date.
Your provider may also move the date. A first-trimester ultrasound measures the embryo directly, and when that measurement disagrees with the period-based estimate by more than a few days, the ultrasound wins — official guidance treats early ultrasound as the most accurate dating method. If that happens, the scan date is the one to trust, not this page.
What the timeline gives you
Alongside the date itself, the results map your trimesters and the appointments that anchor them: the dating scan around 8–12 weeks, the anatomy scan near 20, glucose screening in the late twenties, and the home stretch of more frequent visits from 36. Knowing roughly when each lands makes the calendar feel less like a mystery managed by other people.
Planning in the other direction — trying to conceive and working out your fertile days — is what the ovulation calculator is for, and the pregnancy weight gain calculator covers healthy weight ranges once you're expecting.
Frequently asked questions about due dates
How accurate is a due date calculator?
Only about 4–5% of babies are born on the exact estimated due date. Full term is usually 37–42 weeks. Treat the date as the middle of a window, not a guarantee.
Can my doctor change my due date?
Yes. Early ultrasound dating can shift the estimated due date, especially if cycle length differs from 28 days or the LMP is uncertain.
When should I contact my prenatal provider?
Call your provider for heavy bleeding, severe pain, fever, reduced fetal movement (after you feel kicks), or any symptom that worries you — do not wait for a due date.
Is this medical advice?
No. This tool provides educational estimates only. Prenatal care decisions belong with your doctor, midwife, or OB-GYN.
How do I use this due date calculator?
Choose last menstrual period or conception date, enter the date, and click Calculate to see your estimated due date and milestones.
Sources and further reading
- ACOG — Methods for estimating the due date
- MedlinePlus — Pregnancy due date
- NHS — Pregnancy due date information
This page is for general information only and is not medical advice. Always confirm dates and care decisions with your prenatal provider.