Pregnancy Due Date Calculator

A pregnancy due date calculator provides an estimated date based on established obstetric formulas, but its accuracy depends on the method used and the reliability of the input data. The LMP method using Naegele's rule is accurate within a range of about plus or minus 10–14 days for women with regular 28-day cycles. First-trimester ultrasound dating is the most accurate clinical method, with a margin of plus or minus 5–7 days. IVF-based calculations are the most precise of all methods. Regardless of the method, only about 4–5% of babies are born on their exact estimated due date. Approximately 70% of babies are born within 10 days of the due date, and about 90% arrive within two weeks. The due date is best understood as the midpoint of a likely delivery window rather than a specific birth date.

The due date from LMP is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. An equivalent formulation is to add one year, subtract three months, and add seven days to your LMP date. For example, if your LMP was March 1, your estimated due date would be December 6 (March 1 + 1 year = March 1 next year, minus 3 months = December 1, plus 7 days = December 8 — or simply March 1 + 280 days = December 6). This method assumes a 28-day cycle with ovulation on day 14. If your cycle length differs, the calculation should be adjusted: add or subtract the difference between your cycle length and 28 days. A woman with a 32-day cycle would have her due date shifted 4 days later than the standard calculation.

Naegele's rule is a standard method for calculating the estimated due date of pregnancy, named after German obstetrician Franz Karl Naegele who popularized it around 1812. The rule works by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). The mathematical shortcut is: take the LMP date, add one year, subtract three months, and add seven days. The 280-day figure is based on the assumption that pregnancy lasts approximately 266 days from conception and that ovulation occurs 14 days after the start of menstruation in a 28-day cycle (266 + 14 = 280). While Naegele's rule remains the most widely used dating method in obstetrics and is endorsed by ACOG, it has known limitations — it assumes regular 28-day cycles and does not account for individual variation in ovulation timing, maternal age, or parity.

Yes, your due date can and often does change after an ultrasound, particularly if there is a significant discrepancy between the ultrasound-based date and the LMP-based date. According to ACOG guidelines, the due date should be revised to match the ultrasound dating if the discrepancy exceeds 5 days before 9 weeks of gestation, 7 days between 9 and 13 weeks, 10 days between 14 and 15 weeks, 14 days between 16 and 21 weeks, or 21 days after 22 weeks. First-trimester ultrasound is considered the most reliable method for pregnancy dating because early embryonic growth follows a highly predictable pattern. Once a due date is established by an early ultrasound, it should not be changed by subsequent ultrasounds, as later measurements become less accurate due to normal variation in fetal size.

If you have irregular periods, the standard LMP calculation may be inaccurate because it assumes ovulation occurs on day 14 of a 28-day cycle. Several alternative approaches can help: First, if you tracked ovulation using basal body temperature, ovulation predictor kits, or cervical mucus monitoring, use the conception date method (conception date + 266 days) for a more accurate estimate. Second, if you know your average cycle length, our calculator adjusts the due date accordingly — for example, a 35-day cycle shifts the due date 7 days later than the standard calculation. Third, and most recommended for irregular cycles, request a first-trimester dating ultrasound between 8 and 13 weeks. This measurement of crown-rump length (CRL) is accurate within plus or minus 5–7 days and does not depend on menstrual history. Women with PCOS or other conditions causing very irregular cycles should rely primarily on ultrasound dating.

Gestational age and fetal age (also called embryonic age or conceptional age) measure pregnancy from two different starting points, creating a consistent two-week difference. Gestational age counts from the first day of the last menstrual period (LMP), which is approximately two weeks before conception actually occurs. This is the standard measurement used in clinical practice, pregnancy apps, and due date calculations. A full-term pregnancy is 40 weeks gestational age. Fetal age counts from the actual date of conception (fertilization), making it about two weeks less than gestational age. A full-term pregnancy is 38 weeks fetal age. For example, when your healthcare provider says you are 12 weeks pregnant, they mean 12 weeks gestational age, which corresponds to approximately 10 weeks of actual embryo or fetal development. This distinction matters for understanding developmental milestones and interpreting medical literature about pregnancy.

IVF due dates are calculated based on the embryo transfer date and the developmental stage of the embryo. For a Day 3 embryo transfer, the due date is the transfer date plus 263 days, because the embryo is already 3 days old at transfer (266 days of fetal development minus 3 days = 263 days remaining). For a Day 5 blastocyst transfer, the due date is the transfer date plus 261 days (266 minus 5 = 261 days remaining). To convert to gestational age equivalent, add 14 days to the fetal age at any point. For frozen embryo transfers (FET), the same calculation applies regardless of how long the embryos were frozen — the age of the embryo at the time of freezing determines whether it is Day 3 or Day 5. IVF dating is considered the most accurate method for determining a due date because the exact dates of fertilization, embryo development, and transfer are precisely documented.

Pregnancy is divided into three trimesters, each approximately 13–14 weeks long. The first trimester spans from week 1 through week 13 (LMP-based dating) and covers the period of embryonic development — all major organs form, the heart begins beating around week 6, and the embryo transitions to a fetus at week 8. The second trimester extends from week 14 through week 27, a period of rapid fetal growth when the baby develops fingerprints, hearing, and sleep-wake cycles, and the mother typically begins feeling movement between weeks 18 and 25. The third trimester covers week 28 through week 40 (or delivery), focusing on final maturation of the lungs, brain, and immune system, with significant weight gain — the baby approximately doubles in weight during the last 8 weeks. Near delivery, the terminology becomes more specific: early term is 37–38 weeks, full term is 39–40 weeks, late term is 41 weeks, and post-term is 42 weeks and beyond.

The low percentage of babies born on their exact due date reflects the natural biological variability in pregnancy duration. Several factors contribute to this: First, the due date is calculated based on population averages (280 days from LMP), but individual pregnancy length varies naturally by several weeks depending on genetic factors, hormonal profiles, and fetal development rates. Second, the trigger for labor onset involves a complex cascade of hormonal signals from both mother and baby that cannot be predicted to a specific day. Third, the LMP-based calculation introduces inherent uncertainty because it assumes a standard 28-day cycle with ovulation on day 14, which applies to only about 13% of women. Fourth, even the most accurate dating method (first-trimester ultrasound) has a margin of plus or minus 5–7 days. The normal range for full-term delivery spans from 37 to 42 weeks — a five-week window. About 50% of babies arrive within one week of the due date, and roughly 90% arrive within two weeks.

These terms describe the timing of delivery relative to the due date and have specific clinical definitions established by ACOG and the Society for Maternal-Fetal Medicine. Early term covers 37 weeks 0 days through 38 weeks 6 days — the baby is considered nearly mature but may still benefit from continued development, particularly of the lungs and brain. Elective delivery is generally not recommended during this period unless medically necessary. Full term encompasses 39 weeks 0 days through 40 weeks 6 days and is the optimal delivery window where outcomes for both mother and baby are best. The baby's brain at 39 weeks is approximately 30% larger than at 35 weeks. Late term spans 41 weeks 0 days through 41 weeks 6 days, occurring in about 10% of pregnancies. Close monitoring with non-stress tests and fluid checks is recommended, and induction is often discussed. Post term begins at 42 weeks 0 days, affecting 5–10% of pregnancies. Risks of stillbirth, macrosomia, and meconium aspiration increase significantly, and most providers will recommend delivery to prevent complications.