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Enter the first day of your last menstrual period (LMP) and your average cycle length. The calculator adds 280 days (40 weeks) to the LMP using Naegele's rule to estimate your due date, then counts the completed weeks from the LMP to the reference date, today, unless you pick another date, to show how many weeks pregnant you are and which trimester you are in. Results are estimates for planning only, not medical advice, always confirm dating with your healthcare provider.
The standard obstetric estimate of a due date is Naegele's rule: add 280 days, exactly 40 weeks, to the first day of the last menstrual period. The rule assumes a regular 28-day cycle with ovulation around day 14, which is why pregnancy is dated from the last period rather than from conception, about two weeks later. For cycles longer or shorter than 28 days, clinicians shift the estimate by the difference, a 35-day cycle moves the due date about a week later. Only about 5% of babies arrive on the exact calculated date, so most providers talk about a due month rather than a due day.
Pick the first day of your last period from the date field and enter your average cycle length in days, use 28 if you are unsure. Leave the reference date blank to see how many weeks pregnant you are today, or set it to any date to check where you stood or will stand on that day. The results show your estimated due date, your completed weeks of pregnancy and your current trimester at a glance.
Pregnancy is divided into three trimesters: the first runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 until birth. The first trimester covers major organ development, the second usually brings the first felt movements and the anomaly scan around week 20, and the third is about rapid growth and getting into position for birth. Knowing your week also tells you which prenatal appointments and screening windows are coming up, most dating ultrasounds happen between weeks 8 and 12.
A due-date estimate is a statistical midpoint, not a promise: most healthy pregnancies deliver anywhere from about two weeks before to one or two weeks after the calculated date. Full term is defined as 37 to 42 weeks of gestation, so there is a wide normal window. First-trimester ultrasound dating is more accurate than LMP-based estimates, usually within 5 to 7 days, especially when cycles are irregular or the last-period date is uncertain.
Ovulation typically happens about 14 days before the next period, so conception is estimated as the last period plus (cycle length − 14) days, for a 28-day cycle that is day 14. If you know the actual conception date, add 266 days (38 weeks) for an alternative estimate. Think of this calculator as a baby due date estimator: it applies a small adjustment for your cycle length and maps the milestones of your pregnancy by week. With irregular cycles, neither method is very precise; an early dating ultrasound is the most reliable way to date the pregnancy.
Naegele's rule assumes a 28-day cycle. For a longer cycle, add the difference to your due date, a 35-day cycle shifts it about 7 days later, and for a shorter cycle, subtract the difference. Conception timing also shifts: it is roughly the last period plus (cycle length − 14) days. If your cycles are irregular, ask your provider about an early dating ultrasound, which is more accurate.
It is an estimate, not an exact prediction, only about 1 in 20 babies arrives on the calculated date. Most healthy births fall within roughly two weeks on either side. First-trimester ultrasound dating narrows this to about 5–7 days and is the most accurate method available.
The first trimester runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 until delivery. The calculator above shows your current trimester based on your completed weeks.
Yes, add 266 days (38 weeks) to the date of conception. This works best when you tracked ovulation closely or know the conception day. Pregnancy is conventionally dated from the last period, which is about two weeks before conception, so the two methods should agree.
280 days is exactly 40 weeks, which works out to about nine calendar months plus one week. Doctors count pregnancy in weeks from the last menstrual period because weeks are more precise than months for scheduling tests and monitoring growth.
A full-term pregnancy runs from 37 weeks (259 days) to 42 weeks (294 days) of gestation. Babies born in this window are considered to have completed normal development; births before 37 weeks are classed as preterm.
Count the completed weeks since the first day of your last period, that is your gestational age in weeks. Enter your dates above and the calculator shows it instantly, using today as the reference date unless you choose another.
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