Pregnancy Due Date Calculator (LMP & Conception)

Pregnancy Due Date Guide

How Due Dates Are Calculated?

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculation adjusts accordingly. Only about 5% of babies are born on their exact due date — most arrive within 2 weeks either side. The NHS offers a dating scan at 8–14 weeks which provides the most accurate gestational age estimate.

What should I know about Trimester Breakdown?

First trimester (weeks 1–13): organ development, highest miscarriage risk, many women experience morning sickness. Nuchal translucency screening at 11–14 weeks. Second trimester (weeks 14–26): 'golden period' for many — energy returns, bump becomes visible, anomaly scan at 20 weeks. Third trimester (weeks 27–40): rapid growth, preparation for birth, 28-week growth scan, 36-week check for presentation. Term is defined as 37–42 weeks — under 37 weeks is premature, over 42 weeks is post-term.

What do I need to know about Viability and Key Milestones?

24 weeks: threshold of viability — survival outside the womb is possible with intensive neonatal care. 28 weeks: survival rate approximately 90%. 32 weeks: outcomes very close to full-term. 36 weeks: late preterm — babies often do well but may need some support. 37 weeks: early term. 39–40 weeks: full term — optimal outcome window. 42 weeks: post-term, usually induction is offered. The term 'weeks pregnant' counts from LMP, not conception — so at 4 weeks pregnant, the embryo is approximately 2 weeks old from conception — a common source of confusion when comparing dates between sources.

What's the key thing to understand about IVF Dating?

For IVF pregnancies, the embryo transfer date determines gestational age. A 5-day blastocyst transfer adds 266 days (38 weeks) to the transfer date to calculate the EDD — equivalent to 19 days before the transfer being day 1 of the cycle. A 3-day transfer adds 268 days. IVF dating is generally more precise than LMP dating because the transfer date is exactly known. The first trimester dating scan is still recommended to confirm development.

What do I need to know about Naegele's Rule and Its Accuracy?

The standard method for estimating a due date — adding 280 days (40 weeks) to the first day of the last menstrual period — is known as Naegele's Rule, developed by German obstetrician Franz Naegele in the 19th century. It assumes a 28-day cycle with ovulation occurring on day 14, which is why it is most accurate for people with that exact cycle length, and progressively less accurate the further someone's actual cycle length deviates from 28 days — someone with a consistently 35-day cycle, for example, typically ovulates later in their cycle, meaning conception (and so the actual due date) tends to run later than Naegele's Rule alone would predict. For this reason, when cycle length is known and differs from 28 days, many due date calculators and midwives adjust the estimate accordingly, and an early dating ultrasound scan (typically offered between 10 and 14 weeks in the UK) that measures the fetus directly is considered more accurate than last-menstrual-period dating alone, particularly for anyone with irregular cycles, recent use of hormonal contraception, or uncertainty about their last period date. Once an ultrasound dating scan has been performed, the NHS generally uses that scan-based due date going forward rather than recalculating from the last menstrual period.

What's the key thing to understand about Why Due Dates Are Rarely Exact?

A 'due date' describes the single most statistically likely day of delivery, not a firm appointment, and in practice only a small minority of babies — commonly cited as around 4-5% — are born on their exact calculated due date. Full-term delivery is considered normal across a fairly wide window: the NHS and most international guidance define full term as anywhere from 37 to 42 weeks of pregnancy, meaning a baby born three weeks 'early' or one to two weeks 'late' relative to the due date is both common and medically unremarkable. First-time pregnancies also tend to run slightly longer on average than subsequent ones for the same person. Because of this natural spread, due dates are better understood as the midpoint of a normal range rather than a prediction with any precision narrower than a couple of weeks — which is also why healthcare providers rarely intervene for being 'overdue' until significantly past the due date (typically induction is discussed around 41-42 weeks in the UK, not immediately after day 280). Despite being an estimate, the due date remains clinically useful as the reference point against which fetal growth, antenatal screening timing, and maternity leave planning are all organised.

What should I know about Post-Term Pregnancy?

Pregnancy is considered post-term once it extends beyond 42 completed weeks, and 'late-term' from 41 to 42 weeks — a stage where the risk profile shifts noticeably, which is why most UK NHS trusts routinely offer induction of labour somewhere between 41 and 42 weeks rather than waiting indefinitely for labour to start naturally. The main concerns with pregnancies extending well past the due date include the placenta becoming less efficient at supplying oxygen and nutrients over time, reduced amniotic fluid volume, and a larger baby potentially increasing the risk of a difficult birth. These risks rise gradually rather than appearing suddenly at exactly 42 weeks, which is why care in this period typically includes increased monitoring — such as additional ultrasound scans and fetal heart rate monitoring — if a woman chooses to wait a short additional period before accepting induction, rather than monitoring stopping and risk being treated as negligible until a fixed date. The specific point at which induction is offered, and how monitoring is managed while waiting, is a decision made between a woman and her maternity team based on her individual circumstances, rather than a single rule applied identically to every post-term pregnancy.

Worked Example: Comparing Due-Date Estimates From Different Dating Methods

Because ovulation timing and cycle length vary between women, different dating methods can produce slightly different estimated due dates for the same pregnancy — this is one reason the NHS uses the first trimester dating scan (usually 10–14 weeks) to set or confirm the official due date rather than relying on LMP-based calculation alone, especially for women with irregular cycles.

Worked example: if a woman's last menstrual period began on 6 November, Naegele's Rule adds 280 days to estimate a due date of around 12 August the following year. If she knows she ovulated on day 14 of that cycle (a 28-day cycle), the conception-based method (adding 266 days to the conception date) gives the same estimate, because the two methods are mathematically equivalent for a textbook 28-day cycle. However, if her cycles typically run longer — say 35 days, with ovulation on day 21 rather than day 14 — the LMP-based estimate would be about a week too early, which is exactly the kind of mismatch the dating scan is designed to catch and correct.

What's the Difference Between a Due Date and an EDD on Medical Records?

UK antenatal notes almost always use the abbreviation EDD, standing for Estimated Date of Delivery (sometimes written EDC, for Estimated Date of Confinement, in older records) rather than the word "due date" itself. The two terms mean exactly the same thing in practice, but seeing "EDD" on a scan report or hand-held maternity notes for the first time can understandably cause confusion for expectant parents unfamiliar with the abbreviation. The EDD recorded on official notes is normally set, and then fixed, at the dating scan (usually 10-14 weeks) rather than being recalculated at every subsequent appointment — even if a later scan suggests a baby is measuring slightly larger or smaller than expected, this is usually recorded as a growth observation rather than used to change the already-established EDD.