Pregnancy Due Date Calculator
LiveA typical human pregnancy lasts about 40 weeks measured from the first day of the last menstrual period (LMP), which is the convention obstetricians have used for over a century. The Naegele's rule formula adds 280 days to the LMP to estimate the due date, assuming a standard 28-day cycle and ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, this calculator adjusts. If you know your conception date directly, the due date is 266 days from that point. If you have had an ultrasound that measured the embryo or fetus, the scan-based date is usually the most accurate, especially in the first trimester where size variation between pregnancies is smallest. The calculator shows the estimated due date, your current gestational age in weeks and days, the trimester you are in, and the estimated date of conception. None of these are precise predictions. About 4 percent of births happen exactly on the due date; roughly 90 percent fall within two weeks before or after the calculated date. The number is best treated as a planning anchor, not a deadline.
How the three modes differ
Last menstrual period (LMP). The simplest and most commonly used method. It works because almost all women track their period, and the LMP date is usually known with a few days of accuracy. The downside is that it assumes ovulation on cycle day 14, which is true on average but varies. For women with consistently long or short cycles, the cycle-length adjustment in the calculator improves accuracy.
Conception date. The most accurate input when known, but rarely known exactly. Couples who track ovulation through basal body temperature, ovulation predictor kits, or specific intercourse timing in IVF cycles can sometimes pinpoint this within a day or two. If you know it, this mode produces the most precise due date.
Ultrasound dating. The medical gold standard, especially for scans done between weeks 8 and 13 of gestation. In that window the embryo and fetus develop on a very consistent trajectory across pregnancies, so a measurement of crown-rump length lets the sonographer assign a gestational age accurate to within a few days. Scans later in pregnancy are less accurate because individual growth variation widens.
What the gestational age and trimester mean
Gestational age is the number of weeks and days since the first day of your LMP, even if you do not technically become pregnant until ovulation about two weeks later. This convention dates back to a time when ovulation could not be measured, only the menstrual period. Modern obstetrics has kept the convention for continuity with decades of clinical data, which is why a 'six-week pregnancy' usually corresponds to about four weeks since conception.
The trimesters are simply convenience labels that group the pregnancy into phases with similar clinical milestones. First trimester runs through week 12 (12 weeks 6 days); it covers the highest miscarriage risk and the earliest organ formation. Second trimester runs weeks 13 through 26; usually the most physically comfortable phase. Third trimester runs week 27 to delivery and includes the most rapid fetal growth.
Why the due date is fuzzy
Only about 4 percent of births happen on the calculated due date. About 60 percent fall within a week of it, and about 90 percent within two weeks. The pregnancy is considered full-term from week 39 through 40 weeks 6 days. Births in the 37th and 38th weeks are 'early term' and after week 41 are 'late term.' Obstetric guidelines typically discuss induction at or shortly after 41 weeks if labor has not started naturally, because risks rise after that point.
Factors that can shift the due date include first-baby vs subsequent pregnancies (first-time mothers tend to deliver slightly later on average), maternal age, and various medical factors that an obstetrician will track. The calculator's number is the population-typical estimate; your specific care team will refine it.
When to trust which input
If you are early in pregnancy and have not had an ultrasound yet, use the LMP date. If your cycle is consistently 30 or 32 days rather than 28, set the cycle length accordingly; the result will be a few days later than the standard 280-day formula. Once you have a first-trimester ultrasound, switch to that mode using the dating from the scan; that is the number your obstetrician will use too.
Important: this is not medical advice
The calculator estimates dates. It does not replace prenatal care. Schedule a visit with an obstetrician or midwife early in pregnancy. Many of the most important interventions, including dating scans, screening tests, and folic acid supplementation, are time-sensitive. A calculator can give you an anchor; only your care team can interpret what it means for your specific situation, history, and risk factors. They will also recheck and revise the date during the first major ultrasound, typically between 8 and 13 weeks, when the scan-based dating is more accurate than any calculation from menstrual history alone.
Frequently asked questions
LMP is what almost all women know precisely. Conception is harder to pinpoint without ovulation tracking. The 280-day convention has been used in obstetrics for over a century and produces good estimates for most pregnancies.
Related calculators
- HealthBMI CalculatorCalculate your body mass index in metric or imperial units. See the category breakdown and the healthy weight range for your height.
- HealthSleep CalculatorCalculate the best bedtime or wake time based on 90-minute sleep cycles, so you wake up between cycles and feel refreshed instead of groggy.
- HealthHydration CalculatorCalculate your recommended daily water intake in ml, liters, cups, and 500 ml bottles. Adjusts for activity, climate, and pregnancy.