Due Date Calculator

This calculator estimates your due date using Naegele's rule, a method named after the 19th-century German obstetrician Franz Karl Naegele. The rule is simple: take the first day of your last menstrual period (LMP) and add 280 days, which is 40 weeks. That number comes from an old convention of counting pregnancy as ten lunar months of 28 days each, and it has remained the standard starting point for due date estimation ever since, even though modern obstetrics now leans on ultrasound measurements as a complementary check.

The single most common source of confusion with this method is what the 280 days actually represent. Pregnancy is dated from the first day of your last period, not from the day you actually conceived. Because ovulation and conception typically happen around two weeks after your period starts, your real gestational age from fertilization is usually about two weeks less than the "weeks pregnant" number your due date implies. In other words, when a due date calculator says you are at week 8, you have generally only been carrying the embryo for around six weeks since conception. This is a counting convention, not an error, but it trips up a lot of people the first time they encounter it.

Naegele's rule assumes a textbook 28-day cycle with ovulation occurring around day 14. Real cycles vary quite a bit from person to person, and even from month to month for the same person, which is why this calculator asks for your average cycle length rather than assuming 28 days for everyone. If your cycles run longer than 28 days, ovulation tends to happen later in the cycle, which pushes conception — and therefore the due date — later as well. A 35-day cycle, for example, shifts the estimated due date about a week later than the standard calculation would suggest. Shorter cycles work the other way, moving the estimate earlier. This adjustment brings the estimate closer to reality for people whose cycles don't match the 28-day textbook case, though it still can't fully correct for cycles that are irregular or unpredictable.

Once a due date is estimated, the rest of the pregnancy is commonly divided into three trimesters and a "full term" window, both counted from the same last-period starting point. The second trimester is generally considered to begin around week 13, the third trimester around week 27, and full term around week 37. These are widely used reference points for tracking how a pregnancy is progressing and for anticipating which prenatal tests and appointments are typically scheduled at each stage, though the exact boundaries and the recommended checkup schedule can vary somewhat between healthcare providers and countries.

It's worth being clear-eyed about what a due date actually predicts, which is less than most people assume. Only a small share of babies are born on their exact estimated due date — most research suggests it's a low single-digit percentage. The much more useful way to think about a due date is as the midpoint of a range: most babies arrive somewhere within about two weeks before or after that date, and being born a bit early or a bit late within that window is entirely normal rather than a sign that something is wrong. Due dates are best understood as a planning estimate, not a deadline.

There are also situations where this last-period-based estimate is noticeably less reliable. If your cycles are irregular, if you're not sure exactly when your last period started, if you conceived while breastfeeding or shortly after stopping hormonal birth control, or if you have a condition that affects ovulation timing, the calculation here can be off by more than the usual margin. In these cases, and generally as pregnancy progresses, an early first-trimester ultrasound is often considered a more accurate way to date a pregnancy than menstrual history alone, because it measures the embryo or fetus directly rather than relying on an assumed cycle pattern. Your due date may be revised after that scan, and that's a normal part of prenatal care rather than something to worry about.

This tool is a planning aid, not a medical device, and it does not diagnose, confirm, or monitor a pregnancy. It cannot account for your individual health history, prior pregnancies, or findings from an actual clinical exam. If you think you might be pregnant, if you have irregular cycles, or if you have any questions about your due date or prenatal care, please speak with a doctor, midwife, or other qualified healthcare provider — they can confirm dates with an exam or ultrasound and guide you through the appointments appropriate for your situation.