Pregnancy Due Date Calculator

Naegele's rule is the standard method used to estimate a pregnancy due date from the first day of the last menstrual period (LMP). It assumes a 28-day cycle with ovulation around day 14, and a pregnancy length of 280 days (40 weeks) counted from the LMP — even though conception itself typically happens about two weeks after the LMP, closer to ovulation. This is why "weeks pregnant" at the start can feel confusing: in the first two weeks of a "pregnancy" by this dating convention, conception may not have even happened yet. It is simply the standard, consistent starting point used across obstetric care so that everyone — midwives, doctors, ultrasound technicians — is measuring from the same reference point. The 280-day estimate is a population average, not a guarantee. Actual gestation length varies naturally: full-term births can happen anywhere from about 37 to 42 weeks, and first-time pregnancies statistically run slightly longer on average than subsequent ones. The due date this calculator provides — like the one printed on any standard pregnancy wheel or app — is best understood as "the date around which delivery is most likely," not a fixed appointment. Only a small percentage of babies are actually born on their exact estimated due date. A common misconception is that a due date calculated from LMP is just as accurate as one calculated from an ultrasound measurement, particularly in early pregnancy. In practice, healthcare providers often adjust the estimated due date after a dating ultrasound (typically performed in the first trimester), especially if a person's cycle isn't a textbook 28 days, if the exact LMP date is uncertain, or if ovulation happened notably earlier or later than day 14 of the cycle. If your provider has given you a due date from an ultrasound that differs from the LMP-based estimate, the ultrasound-based date is generally considered more reliable for that individual pregnancy. Worked example: an LMP of 1 January 2026 gives an estimated due date of 8 October 2026 (1 January + 280 days). If today's date is 18 July 2026, that is 198 days after the LMP, or 28 completed weeks — placing the pregnancy at the very start of the third trimester (weeks 28+). This tool is provided for general informational and planning purposes — tracking approximate milestones, estimating trimester transitions, or getting a rough sense of timing — and is not a substitute for professional prenatal care. Due dates, week counts, and trimester boundaries should always be confirmed with a qualified healthcare provider, who can incorporate ultrasound measurements, cycle history, and other clinical information this simple calculation cannot account for.