Due Date Calculator
Estimate when your baby will arrive using the same method clinicians use — Naegele's rule, with adjustments for cycle length.
Default 28 days. The calculator adjusts if your cycle is shorter or longer.
How this calculator works
Pregnancy is conventionally measured in gestational weeks from the first day of your last menstrual period (LMP) — not from conception. This is because most people know their period dates more precisely than the day they actually conceived.
Naegele's rule
The standard formula is: LMP + 280 days = due date. That's 40 weeks. The 280 figure was published by German obstetrician Franz Naegele in 1812 and remains the clinical default.
Cycle-length adjustment
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, ovulation — and therefore conception — happens earlier or later. We adjust the due date accordingly: LMP + 280 + (cycle - 28) days.
Conception-date method
If you know your exact conception date (IVF transfer, fertility tracking with confirmed ovulation), the math is simpler: conception + 266 days. The 14-day difference reflects that gestational age starts before conception.
Why your due date may shift
Your provider will likely confirm your due date with a first-trimester ultrasound, ideally before 13 weeks. The crown-rump length of the embryo at that stage gives a very accurate gestational age — within ±3 days. If the ultrasound disagrees with your LMP estimate by more than about a week, the ultrasound usually wins.
What happens if you go past your due date?
It's normal. About 5% of babies arrive on their exact due date; most arrive within two weeks before or after. Pregnancies are typically considered full term at 39 weeks and post-term at 42+ weeks. If you reach 41 weeks without labor, your provider will discuss induction options.
Frequently asked questions
How accurate is a due date calculator?
A due date calculator using Naegele's rule is a solid first estimate, but only ~5% of babies arrive on their actual due date. Most arrive within two weeks before or after. Your first-trimester ultrasound (especially before 13 weeks) is the most accurate way to confirm your due date.
How is my due date calculated?
The standard method is Naegele's rule: take the first day of your last menstrual period (LMP) and add 280 days (40 weeks). For irregular cycles, we adjust by adding the difference between your cycle length and 28 days.
What if I don't remember my last period?
Use the conception-date method (tab above) if you tracked ovulation or had IVF. Otherwise, your healthcare provider can date your pregnancy with an early ultrasound — measuring the embryo gives a very accurate estimate up to about 13 weeks.
Why does my due date matter?
It anchors your prenatal care schedule, screening tests, fetal-development milestones, and helps your provider decide if labor needs to be induced if you go past 41-42 weeks.
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