Pregnancy Due Date Calculator

What should I know about 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.

What do I need to know about 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, which is why care providers generally avoid inducing before 39 weeks without medical reason.

What should I know about Why Most Babies Don't Arrive on Their Due Date?

A due date is a single calculated point, but birth timing naturally falls across a range rather than landing precisely on that date for most pregnancies. 'Full term' is defined as anywhere from 37 to 42 weeks of pregnancy, a five-week window, reflecting that normal, healthy variation in exactly when labour begins is wide. The majority of babies are born within the fortnight surrounding the calculated due date rather than on the date itself, and being a few days or even a couple of weeks either side of the due date, within the full-term window, is entirely normal rather than a sign anything is wrong. This is partly because Naegele's rule and similar dating methods estimate an average gestation length, but individual pregnancies vary around that average due to factors including the mother's own cycle length, genetics, and whether it's a first or subsequent pregnancy (first pregnancies statistically run slightly longer on average). This is why NHS antenatal care frames the due date as a planning estimate rather than a prediction to expect precisely, and why induction of labour isn't typically offered until pregnancy extends meaningfully past the due date, usually around 41-42 weeks, rather than at the due date itself.

What's the difference between Ultrasound Dating and LMP Dating?

Due dates can be calculated two main ways, and they don't always agree. LMP dating uses the first day of the last menstrual period plus a standard 280-day (40-week) estimate, assuming a textbook 28-day cycle with ovulation on day 14 — an assumption that doesn't hold for everyone, since natural cycle length varies considerably between individuals and sometimes cycle to cycle in the same person. Ultrasound dating, typically done at the first trimester dating scan (around 8-14 weeks in the UK), measures the size of the developing baby directly (usually crown-rump length at this stage) and compares it against standardised growth charts to estimate gestational age, which doesn't depend on the mother accurately recalling her last period or having a regular cycle. Because of this, UK NHS practice uses the dating scan measurement to set the official due date whenever it differs meaningfully from the LMP-based estimate, treating ultrasound dating as more reliable, particularly for women with irregular cycles, uncertain last period dates, or conception via fertility treatment where exact timing is already known. This is why someone's official due date may shift slightly after the first scan compared to an initial LMP-based calculation done earlier in the pregnancy.

Worked Example: Trimester Dates From a Sample LMP

Once a due date is estimated, the standard pregnancy trimester boundaries (first trimester: weeks 1-12, second trimester: weeks 13-27, third trimester: weeks 28-40) can be converted into real calendar dates for a specific pregnancy, which many parents find more useful than the week numbers alone for planning antenatal appointments and milestones.

Worked example: for a last menstrual period of 6 November, the estimated due date (LMP + 280 days) is around 12 August. Counting forward, the first trimester runs from 6 November to roughly 29 January (end of week 12), the second trimester from 30 January to roughly 14 May (end of week 27), and the third trimester from 15 May through to the due date around 12 August. These date ranges shift correspondingly for any other LMP date, which is why apps and calculators convert week-based milestones into a personalised calendar rather than leaving expectant parents to count weeks manually.