Your due date is a five-week window

A due date is presented as a single day, which is the most misleading thing about it. It is one day selected from a five-week range of normal outcomes, calculated from an assumption about your cycle that may not hold, and only about 4% of babies actually arrive on it. Understanding how the number is produced makes it far more useful than treating it as a deadline.

Naegele's rule: 280 days and one big assumption

The standard method adds 280 days — 40 weeks — to the first day of your last menstrual period. Last period starting 5 January 2026 gives a due date of 12 October 2026.

The assumption buried in that is a 28-day cycle with ovulation on day 14. If your cycle is longer, ovulation happened later, so conception happened later and the pregnancy is younger than the date implies. The correction is the difference from 28: a 32-day cycle adds four days, moving the same example to 16 October 2026. A 24-day cycle would pull it four days earlier. Four days sounds trivial until you are being offered an induction for being "overdue".

Two ways of counting age, two weeks apart

Pregnancy is measured in gestational age, counted from the last period. Embryonic age is counted from conception, roughly two weeks later. So at 8 weeks gestational the embryo has been developing for about 6 weeks.

Clinical care, scan reports and every week-by-week guide use gestational age. Apps and articles occasionally use the other, and a two-week discrepancy is alarming when you do not know which convention you are reading. If a figure looks two weeks off, this is almost always why.

The window that actually matters

Term is not a single date. From that 5 January example with a 32-day cycle, 37 weeks falls on 25 September and 42 weeks on 30 October — a 35-day span within which arrival is considered normal. Early term, full term and late term all sit inside it.

Converting gestational weeks into months is its own small annoyance, since a pregnancy is nine months and forty weeks and those do not divide neatly — 40 weeks is closer to nine and a bit. Read the due date as the centre of that window rather than a target. It is a scheduling reference for monitoring, not a prediction, and treating it as a prediction is what makes the last fortnight of a pregnancy feel like being late for something.

Why a scan may overrule the arithmetic

An early ultrasound measures the fetus directly and, in the first trimester, dates a pregnancy more accurately than the calendar does — because it does not have to assume anything about your cycle or when you ovulated. If a scan date and a period-based date disagree by more than a few days, clinicians generally revise to the scan.

So a calculator like ours is genuinely useful before your first scan and as a sanity check afterwards. It is not a substitute for the dating your midwife or doctor records, and where the two differ, theirs is the one your care is planned around.

What the calculator cannot know

It cannot know when you actually ovulated, whether your recalled period date is exact, or whether this pregnancy will run long or short — and cycle length varies month to month even for people who describe themselves as regular. If you conceived through IVF the dating is different again, worked from the transfer date rather than a period, and the arithmetic here does not apply.

None of that makes the estimate worthless. It makes it an estimate with a stated method, which is the most any date-based calculation can be.