HealthScorer

Pregnancy Due Date Calculator

Estimate your pregnancy due date from the first day of your last period or the date of conception. Uses Naegele's rule (LMP + 280 days) per ACOG and.

Last updated: Sources verified:

How the calculation works

The calculator implements Naegele’s rule, the standard method used by ACOG, NHS, NICE, and most national obstetric bodies:

Estimated due date = first day of last menstrual period (LMP) + 280 days

Or: date of conception + 266 days

For non-28-day cycles, the LMP method adds (cycle length − 28) days, since ovulation is shifted later when cycles are longer.

Trimesters

WeeksTrimester
1 – 13First
14 – 27Second
28 – 40+Third

What this tool isn’t

This is a dating estimate, not a clinical assessment. ACOG’s 2017 committee opinion is explicit that first-trimester ultrasound (preferably between 8 and 13 weeks) is the most accurate method to establish or confirm the estimated due date when the LMP is uncertain — and the ultrasound estimate should be used over the LMP when the discrepancy exceeds defined thresholds.

If you’re using this calculator for planning purposes, that’s exactly what it’s good for. For clinical decisions about labour induction, fetal growth assessment, or anything that depends on precise gestational age, your obstetric care provider’s dating (informed by your scan) takes precedence.

Privacy

All calculation happens in your browser. We never see, log, or store the dates you enter. Anonymous, aggregate events (e.g. which trimester you’re in) are sent to a privacy-first analytics service.

Frequently asked questions

How accurate is a due date based on LMP?
Naegele's rule (LMP + 280 days) gives an estimate, not a fixed appointment. Only about 4% of babies arrive on the calculated date; roughly 70% are born within 10 days of it. First-trimester ultrasound (especially the crown-rump length measurement at 8–13 weeks) is more accurate when the date of conception is uncertain or cycles are irregular — ACOG recommends using ultrasound dating in those cases.
What if my cycle isn't 28 days?
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer, ovulation is later, so we add the difference (cycle length − 28) to the due date. The conception-date method bypasses this entirely — if you know roughly when you conceived, the calculator simply adds 266 days.
What does 'weeks pregnant' mean?
Pregnancy is counted in weeks since the first day of your last period, not since conception. So at the moment of conception you're already considered roughly 2 weeks pregnant. A typical pregnancy lasts 40 weeks measured this way, with the first trimester ending around week 13 and the second around week 27.
When should I see a clinician?
In most countries the first prenatal visit is between weeks 8 and 12, partly to confirm the due date with ultrasound. If you have any pregnancy-related concerns (bleeding, severe pain, decreased fetal movement after week 24), contact your obstetric care provider promptly. This calculator is informational and does not replace clinical assessment.
Does my data leave my device?
No. The calculation runs entirely in your browser. We do not transmit, store, or log the dates you enter. Anonymous usage events (e.g. trimester reached) are sent to a privacy-first analytics service — no personally identifiable information is collected.

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Sources

  1. Methods for estimating the due date — Committee Opinion 700 — American College of Obstetricians and Gynecologists (ACOG) (guideline, retrieved 2026-04-27)
  2. Working out your due date — NHS (gov health, retrieved 2026-04-27)
  3. Antenatal care — NICE guideline NG201 — National Institute for Health and Care Excellence (NICE) (guideline, retrieved 2026-04-27)