Pregnancy Due Date Calculator
Estimate your due date from your last period, conception date, or IVF transfer — plus your current gestational age and trimester.
Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, or conception date plus 266 days — an estimate, not a deadline.
How pregnancy dating works
Obstetric dating counts from the first day of the last menstrual period (LMP) — about two weeks before conception — so "40 weeks pregnant" means 40 weeks of gestational age, not 40 weeks since fertilisation. The classic rule, described in ACOG's guidance on estimating the due date, adds 280 days to the LMP. Known conception shifts the math to +266 days, and IVF transfers are the most precisely dated of all: +263 days for a day-3 embryo, +261 for a day-5 blastocyst, straight from the ACOG committee opinion.
One thing this calculator can't do is see your pregnancy. An early ultrasound measures the embryo itself, which is why a scan-based due date from your care provider always takes precedence over any date calculated here.
Pregnancy stages by gestational age (ACOG)
| Stage | Gestational age |
|---|---|
| First trimester | 0w0d – 13w6d |
| Second trimester | 14w0d – 27w6d |
| Third trimester | 28w0d – delivery |
| Preterm birth | Before 37w0d |
| Early term | 37w0d – 38w6d |
| Full term | 39w0d – 40w6d |
| Late term | 41w0d – 41w6d |
| Postterm | 42w0d and beyond |
Trimesters per ACOG's patient guidance; term categories per ACOG Committee Opinion 579.
An estimate, honestly labelled
Whichever method you use, treat the date as the middle of a window rather than an appointment. Only around one baby in twenty-five arrives on the exact day, and healthy pregnancies naturally span a range of several weeks. LMP dating also leans on memory and regular cycles — hormonal contraception shortly before conceiving, or irregular cycles, make the ultrasound comparison that much more important. And a plain-language reminder that matters more than any date: if you have bleeding, severe pain, or reduced movement, contact your care provider straight away rather than consulting a calculator.
Sources: ACOG Committee Opinion 700 (methods for estimating the due date); ACOG Committee Opinion 579 (definition of term pregnancy); ACOG patient guidance on fetal growth; Mongelli et al., Am J Obstet Gynecol (1996); Jukic et al., Human Reproduction (2013). Educational only — not prenatal care or medical advice.
Frequently asked questions
How is a due date calculated?
The convention (ACOG) is 280 days — 40 weeks — after the first day of your last menstrual period, known as Naegele's rule. If you know the conception date, it's that date plus 266 days. For IVF, ACOG specifies transfer date plus 263 days for a day-3 embryo or plus 261 days for a day-5 blastocyst.
What if my cycles aren't 28 days?
Standard LMP dating assumes ovulation on day 14 of a 28-day cycle. A common clinical-calculator adjustment shifts the due date by the difference — for a 33-day cycle, five days later. With irregular cycles the LMP method gets unreliable, which is one reason clinicians confirm dating with a first-trimester ultrasound.
How accurate is a due date?
A due date is a statistical midpoint, not a deadline. Studies report only about 4% of babies arrive exactly on the due date, and roughly 70% within 10 days of it — even when dated by ultrasound. In one cohort with known ovulation dates, natural pregnancy length varied across about five weeks.
Why does my scan date differ from this calculator?
An ultrasound in early pregnancy measures the embryo directly, making it the most accurate dating method — ACOG, AIUM and SMFM all say a first-trimester ultrasound-established due date should be used when it differs meaningfully from the period-based estimate. Whatever your clinician's ultrasound says overrides any calculator.
What do "full term" and the trimesters mean?
ACOG defines the trimesters as 0–13w6d, 14w0d–27w6d, and 28 weeks to delivery. Around the due date, deliveries are classified as early term (37w0d–38w6d), full term (39w0d–40w6d), late term (41w0d–41w6d) and postterm from 42 weeks — categories that guide monitoring near the end of pregnancy.
Sources
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date (Obstet Gynecol, 2017) — ACOG methods for estimating due date
- Baskett & Nagele — Naegele's rule: a reappraisal (BJOG, 2000) — Naegele's rule LMP due-date formula
- Jukic et al. — Length of human pregnancy and its natural variation (Hum Reprod, 2013) — Gestation length from conception/ovulation dating
- NHS — Ultrasound scans in pregnancy (dating scan) — Due date from LMP, dating-scan adjusted
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.