Due Date Calculator (LMP or conception, with trimester)
Pick LMP (last menstrual period) or conception date and see the estimated due date, current gestational age in weeks, days remaining, and current trimester.
How it works
How due date is calculated
The standard estimate (Naegele's rule, 1812) adds 280 days to the first day of the last menstrual period (LMP), assuming a 28-day cycle and ovulation on day 14. From a known conception date, the same due date is 266 days out (280 minus the 14 days from LMP to ovulation).
Pregnancy is conventionally counted in 'gestational weeks' from LMP, not from conception, even though conception occurs about two weeks later. So 'six weeks pregnant' usually means six weeks since LMP, or about four weeks since conception.
Why your due date is an estimate
Only about 4% of babies arrive on the calculated due date. The 280-day formula assumes a 28-day cycle; real cycles range from 21 to 35 days, and ovulation timing varies even within a person from cycle to cycle. Most births fall in the two weeks before or after the calculated date — anywhere from week 38 to week 42.
First-trimester ultrasound dating (weeks 8-13) is more accurate than LMP-based dating, especially for irregular cycles. If you've had an early scan, your healthcare provider's recorded due date is more reliable than this calculator's number.
Trimester boundaries
First trimester: weeks 1-13 — embryo to fetus transition, organ formation. Second trimester: weeks 14-26 — fetal movement, anatomy scan, viability emerges around week 24. Third trimester: weeks 27-40+ — rapid growth and lung maturation. Many guidelines treat 37-42 weeks as 'full term', with births before 37 weeks classified as preterm.
Frequently asked questions
›Why 280 days?
Naegele's rule: 280 days ≈ 40 weeks from LMP. It assumes a 28-day cycle with ovulation on day 14, which gives ~266 days from conception (about 38 weeks).
›What if my cycle is longer or shorter?
The standard formula assumes 28 days. For longer cycles, the actual due date is later by (cycle length − 28) days; for shorter cycles, earlier. Talk to a clinician for an adjusted estimate.
›Is the conception date the same as 'date I had sex'?
Not quite — sperm can survive 3-5 days, so conception can happen up to 5 days after intercourse. If you don't know the exact ovulation date, LMP is more reliable.
›How accurate is this calculator?
It uses the same formula as most clinical apps. The accuracy of any due date is limited by the underlying biology, not the math: only ~4% of babies arrive on the calculated date.
›What's an ultrasound due date?
Early ultrasound (8-13 weeks) measures the fetus and back-calculates the due date. It's more accurate than LMP for people with irregular cycles. If your scan-based due date differs, trust that one.
›Does this calculator know if I had IVF?
No. For IVF, due date = embryo transfer date + 263 days (3-day embryo) or + 261 days (5-day blastocyst). Use the conception mode and adjust manually if needed.
›Can the due date change during pregnancy?
Yes, especially after the first ultrasound. Providers may revise the due date based on growth measurements.
›Is the data sent anywhere?
No — calculation runs locally in your browser.
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