Pregnancy Due Date Calculator

Naegele's Rule

The standard way to estimate a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The logic dates back to a 19th-century German obstetrician and remains the basis of due-date estimation today. The method assumes a regular 28-day cycle with ovulation around day 14, so the 40 weeks are counted from the LMP rather than from conception (which is roughly two weeks later — meaning 'weeks pregnant' counts from before you actually conceived, a common source of confusion). Because not everyone has a 28-day cycle, this calculator adjusts for cycle length: if your cycle is longer than 28 days you ovulate later, pushing the due date back; if shorter, it moves forward. For example, a 32-day cycle adds roughly four days to the estimate. The due date is genuinely an estimate of the middle of a likely range, not a deadline — pregnancy is considered 'term' anywhere from 37 to 42 weeks. If you know your conception date (for instance with IVF or careful cycle tracking), dating can be more precise. The LMP method is simply the most widely available starting point, used when earlier scan dating isn't yet available.

How Accurate Is It?

A due date is a useful target but a poor prediction of the actual birth day — only about 4% of babies arrive on their exact estimated due date. Most births happen within roughly two weeks either side, and a first baby in particular often arrives a little later than the estimate. This is why 'term' covers a five-week window (37 to 42 weeks): a baby born at 38 weeks and one born at 41 weeks are both perfectly normal. The most accurate dating comes from an ultrasound scan in the first trimester, typically the 'dating scan' around 8 to 14 weeks, which measures the baby's size (crown-rump length) to estimate gestational age. If the scan date differs significantly from the LMP-based date, maternity services usually adopt the scan date, as early measurements are very consistent between pregnancies. Later scans are less reliable for dating because babies grow at increasingly different rates. Factors like irregular cycles, recent hormonal contraception, or uncertainty about the LMP all reduce the accuracy of the calculated date — which is exactly when a dating scan becomes most valuable. Treat the calculated due date as a helpful planning estimate, and expect the real event to fall somewhere in the surrounding weeks.

Trimesters at a Glance

Pregnancy is divided into three trimesters, each with characteristic developments. The first trimester (weeks 1 to 12) covers conception, implantation, and the formation of all the baby's major organs — a critical period when early-pregnancy symptoms like nausea and tiredness are common, and when the dating scan usually takes place (around 8 to 14 weeks). The second trimester (weeks 13 to 26) is often the most comfortable phase, with early symptoms easing; the anatomy scan (around 18 to 22 weeks) checks the baby's development in detail, and many people first feel movement during this time. The third trimester (weeks 27 to 40, and sometimes beyond) is when the baby grows most rapidly and prepares for birth, with more frequent antenatal checks to monitor wellbeing as the due date approaches. Key milestones in UK care typically include the dating scan, the anatomy scan, and regular midwife appointments throughout, with extra monitoring if needed. The exact schedule and which scans are offered vary by location and individual circumstances. This calculator estimates your due date and the timing of these stages, but your midwife or maternity team provides the personalised care, dating, and screening that matter — always follow their guidance, and contact them promptly with any concerns during pregnancy rather than relying on any calculator.

What the Due Date Actually Means

Only 4% of babies are born on their estimated due date. The majority of term pregnancies deliver between 38 and 42 weeks, with the average first-time mother delivering at 40 weeks and 5 days. The EDD is the midpoint of a normal distribution, not a deadline. Late-term induction (41-42 weeks) is offered in most UK hospitals as complications begin to increase slightly beyond 42 weeks. Conversely, early deliveries between 37-38 weeks have higher rates of complications than deliveries at 39-40 weeks