Pregnancy Due Date Calculator

Most US providers use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the actual due date may shift by a few days. A first-trimester ultrasound (typically more accurate than LMP) is often used to confirm or adjust the date.

Naegele's rule, formalized by German obstetrician Franz Naegele in 1812, is the standard pregnancy dating formula: LMP date + 1 year − 3 months + 7 days = estimated due date. It's mathematically equivalent to adding 280 days. The rule remains the obstetric standard in the US despite its 28-day-cycle assumption, because actual delivery only correlates loosely with calculation method — most babies arrive in a window around the calculated date.

Only about 4–5% of babies are born on their exact due date. About 80% are born within two weeks of it (between 38 and 42 weeks gestation). First-trimester ultrasound dating is accurate to within ±5 days; LMP-based dating is accurate to within ±2 weeks when LMP is reliably known. Your provider may adjust the official due date after early ultrasound if it differs significantly from LMP.

Use LMP if you know it — that's what providers use as the standard. Conception date is harder to pinpoint exactly because conception can happen days after intercourse (sperm survives 3–5 days in the reproductive tract). If you tracked ovulation precisely (LH testing, IVF transfer date), conception-based dating can be slightly more accurate, but most clinical milestones (anatomy scan, glucose testing, term/post-term cutoffs) are scheduled by LMP-based gestational age.

If your LMP is unknown or irregular, your provider will use ultrasound dating at your first prenatal visit. A crown-rump-length measurement between 7 and 13 weeks is the most accurate dating tool available — better than LMP for irregular cycles or unsure dates. After 13 weeks, ultrasound dating becomes progressively less accurate as fetal growth variation increases.

Gestational age is measured from the first day of LMP — it's what providers and this calculator use, so '12 weeks pregnant' means 12 weeks from LMP. Fetal age (conception age) measures from estimated conception, typically 2 weeks earlier. So 12 weeks gestational = 10 weeks fetal. The discrepancy confuses many people; clinical care universally uses gestational age.

Standard US prenatal schedule: every 4 weeks until 28 weeks, every 2 weeks from 28 to 36 weeks, and weekly from 36 weeks until delivery. Higher-risk pregnancies have more frequent visits. Key tests fall around specific weeks: first ultrasound (8–12), genetic screening (10–13), anatomy scan (18–22), glucose tolerance (24–28), Group B strep (35–37). Total visits: typically 12–15 for a low-risk pregnancy.

Week 6: fetal heartbeat detectable on ultrasound. Week 12: end of the first trimester, miscarriage risk drops significantly. Week 14: many people start showing. Week 16–22: anatomy scan reveals sex if desired. Week 24: viability — babies born from this point have increasing survival odds with NICU care. Week 28: third trimester begins. Week 37: full term (officially 'early term' 37–38 weeks, 'full term' 39–40). Week 40: estimated due date. Week 42: post-term, induction typically discussed.

The American College of Obstetricians and Gynecologists revised terminology in 2013. Early term: 37 weeks 0 days to 38 weeks 6 days. Full term: 39 weeks 0 days to 40 weeks 6 days (this is the ideal birth window). Late term: 41 weeks 0 days to 41 weeks 6 days. Post-term: 42 weeks 0 days and beyond. Babies born even one week early have measurably higher complication rates than full-term babies, which is why elective deliveries before 39 weeks are generally avoided.

Going slightly past the due date is normal. Most US providers will offer membrane sweeping at 39–40 weeks and consider induction between 41 and 42 weeks depending on local protocol and your health. After 42 weeks, induction is strongly recommended because stillbirth risk rises modestly past this point. Discussing your provider's specific induction policy at the 36–38 week visit is reasonable so there are no surprises.