Pregnancy Due Date Calculator

A due date is a single calendar day, but in practice it marks the middle of a roughly four-week window in which most babies actually arrive. Health professionals describe pregnancy as lasting about 40 weeks, or 280 days, yet that clock starts on the first day of your last menstrual period rather than on the day of conception. Because ovulation and conception usually happen around two weeks after that period begins, a "40-week" pregnancy is really about 38 weeks measured from conception. The due date is therefore best understood as the centre of a range: a full-term pregnancy runs from roughly 37 to 42 weeks, and only a small fraction of babies are born on the exact date this calculator produces. Treat the number as a planning anchor for appointments, maternity leave, and nursery prep — not as a deadline your body is obliged to meet.

This calculator applies Naegele's rule, a method obstetricians have used for nearly two centuries because it is simple enough to do in your head and reliable enough for everyday estimates. The rule adds 280 days (40 weeks) to the first day of your last period, then shifts that total if your cycle is longer or shorter than the textbook 28 days. The 280-day figure is not arbitrary: it reflects the average human gestation counted from the last period. The adjustment for cycle length matters because the rule assumes ovulation occurs about 14 days before your next period; when your cycle runs long or short, that ovulation point moves, and the due date moves with it. Entering your real cycle length is the single biggest thing you can do to make the estimate fit your body rather than the statistical average. If your cycle is irregular, use the adjusted-cycle-length option for a more personal estimate than the standard 280-day assumption, and confirm the date with an early ultrasound. Most people can recall the first day of their last menstrual period far more reliably than the day they conceived, so medicine uses that date as the universal starting line. Conception is genuinely hard to know exactly: sperm can survive several days after intercourse, and the timing of ovulation can vary from one cycle to the next. Rather than guess at conception, clinicians count forward from a date you can remember. The calculator still reports an approximate conception date — typically about 14 days before your next expected period — because it is useful context, but it is derived from the period date, never the other way around. If you know your conception date from fertility tracking, you can use it to sanity-check the estimate, but the period-based date remains the standard your care team records. Alongside the due date, the calculator shows when your first and second trimesters end. These are not arbitrary milestones: each trimester brings a different set of developments, screenings, and appointments. The first trimester, through about week 13, covers the earliest and most rapid organ development, which is why many people wait until its end to share the news. The second trimester, roughly weeks 14 to 27, is often the most comfortable and is when the anatomy scan and routine blood tests are usually scheduled. Knowing these boundaries helps you anticipate what is coming and gives your care team a shared timeline. The dates are fixed offsets from your last period, so they track your personal due date rather than a generic 40-week grid. In the first trimester, an ultrasound that measures the embryo is the most accurate way to establish gestational age — often more precise than a period-based estimate, especially if your cycles are irregular or you were not tracking them. If the scan's dating differs from this calculator by more than a few days, your clinician will usually reset the due date to match the measurement, because early fetal size is a better clock than memory of a period. That does not make the estimate useless; it makes it a strong starting point a scan can refine. Use this tool to get oriented before an appointment, to understand why a date might shift, and to plan around the trimesters — then let your care team confirm the official date.

The numbers here are estimates built from a simple rule, not a medical diagnosis. They are most useful for planning and for understanding the language your clinician uses, not for predicting the exact day. If your periods are very irregular, if you became pregnant through fertility treatment, or if you have any uncertainty about your last period, tell your provider so they can date the pregnancy accurately. And whatever any calculator says, contact a healthcare professional as soon as you suspect you are pregnant: early confirmation and prenatal care are what genuinely protect the health of both you and the baby. Let this page be the first step in getting informed, and your clinician be the source of truth.