Due Date Calculator

The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. For cycles other than 28 days, adjust by the difference: a 32-day cycle adds 4 days, making the EDD 284 days from LMP. The 280-day figure assumes ovulation on day 14 of a 28-day cycle, so conception occurs around day 14 and pregnancy lasts 266 days from conception (38 weeks).

LMP-based due dates are accurate to within about 1 to 2 weeks for women with regular 28-day cycles. An early first-trimester ultrasound (8 to 12 weeks) is the gold standard, accurate to plus or minus 5 days. Second-trimester ultrasounds (13 to 26 weeks) are accurate to plus or minus 10 to 14 days. LMP dating becomes less reliable with irregular cycles or uncertain period dates.

Gestational age counts from the first day of the last menstrual period (LMP) and is used by all doctors and ultrasound reports. Fetal age (embryonic age) counts from conception, which typically occurs 14 days after LMP. At 10 weeks gestational age, the embryo is approximately 8 weeks old by fetal age. Always use gestational age (from LMP) when communicating with your healthcare provider.

Yes. Use either the Conception Date mode (if you know when conception likely occurred) or the Ultrasound mode (if you have had a scan with a gestational age measurement). The conception date method adds 266 days to the estimated conception date. The ultrasound method uses the gestational age at the scan date to project forward to 280 days from LMP.

Naegele's rule was proposed by German obstetrician Franz Karl Naegele in 1806: add 1 year, subtract 3 months, and add 7 days to the LMP. This is equivalent to adding 280 days (40 weeks). It was based on the observation that most pregnancies last approximately 10 lunar months (of 28 days each). Although the rule is over 200 years old, it remains the standard clinical dating method worldwide because it is simple and reliable for women with regular cycles.

The main milestones by gestational week are: Week 6-8 (heartbeat detectable on ultrasound), Week 12-13 (end of first trimester, miscarriage risk drops significantly), Week 18-20 (anatomy scan to check fetal development), Week 24 (viability threshold, when survival outside the womb becomes possible), Week 27 (end of second trimester), Week 37 (early term, lungs typically mature), Week 39-40 (full term, optimal delivery window), Week 42 (post-term, induction typically recommended).

Pregnancy is divided into three trimesters. The first trimester spans weeks 1 to 12 (3 months), covering early embryonic development. The second trimester spans weeks 13 to 26, during which the fetus grows rapidly and the pregnant person typically feels more energy. The third trimester spans weeks 27 to 40 (and beyond), with the fetus reaching full size. Most complications occur in the first trimester; the second trimester is often called the most comfortable period.

Cycle length shifts the estimated ovulation date. The LMP-to-ovulation gap equals cycle length minus 14 days (the luteal phase is consistently 14 days). A woman with a 32-day cycle ovulates around day 18, four days later than a 28-day cycle woman. This shifts conception (and thus the due date) by the same 4 days. Naegele's rule assumes 28-day cycles, so this calculator adds or subtracts the cycle-length adjustment automatically.

Full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days (per the American College of Obstetricians and Gynecologists, 2013). Early-term is 37 to 38 weeks; late-term is 41 weeks; post-term is 42 weeks or beyond. Babies born before 37 weeks are considered preterm. The highest fetal readiness across all organ systems occurs in the full-term window, which is why induction or elective cesarean before 39 weeks is generally discouraged for uncomplicated pregnancies.

Add 266 days (38 weeks) to the estimated conception date. Since conception typically occurs around day 14 of a 28-day cycle, this is equivalent to LMP + 280 days (40 weeks). Example: conception on February 14 gives an EDD of February 14 + 266 days = November 7. Use the Conception Date mode in this calculator for an instant result. Note that pinpointing the exact conception date is difficult; a window of plus or minus 2 to 5 days is typical.

Going past the EDD is common. About 50% of first-time mothers deliver after their due date. At 41 weeks, your provider will typically increase monitoring (non-stress tests, amniotic fluid checks). Most guidelines recommend induction by 41 to 42 weeks to reduce the risk of stillbirth and complications associated with post-term pregnancy. A post-term baby (42+ weeks) is at higher risk of meconium aspiration, macrosomia, and placental insufficiency.