Due Date Calculator

Naegele's rule, in plain terms

Pregnancy is conventionally counted from the first day of the last menstrual period, not from conception, because the period is a date most people know while conception usually is not. Naegele's rule adds 280 days — forty weeks — to that date: a last period starting 5 January 2026 gives an estimated due date of 12 October 2026. The rule assumes a regular 28-day cycle with ovulation around day 14, which is why the calculator also shows an approximate conception date two weeks after the LMP.

The 280 days are counted in weeks because pregnancy care is organised by week: the first trimester runs to around week 13, the second to around week 27, and the third from week 28 to birth. The calculator marks the second trimester at LMP + 91 days and the third at LMP + 189 days, both labelled as approximations, since clinicians differ slightly on the exact boundaries.

Why the due date is an estimate

Only a small minority of babies are born on their due date — most arrive within about two weeks either side, and first babies tend to arrive a little late. The rule's assumptions fail often: cycles longer or shorter than 28 days shift ovulation, and a misremembered period date shifts everything by the same amount. That is why an early ultrasound scan, usually offered around 11–14 weeks, is treated as the more accurate dating method and can move the due date by days or even weeks.

If your cycles are irregular, or you conceived with fertility treatment, the LMP rule is a weaker starting point still: with IVF, for example, the transfer date gives a far more precise anchor than any period date. In all of these cases the calculator's result is a first estimate to take to a midwife or doctor, not a date to plan around rigidly.

What the weeks mean in practice

The due date anchors the whole antenatal timetable: screening tests are offered in specific week windows, maternity leave and pay are planned around it, and “term” itself is a range — from 37 weeks to 42 weeks — rather than a single day. A baby born at 39 weeks and one born at 41 weeks are both full-term, which is another reason the exact date matters less than the window around it.

If you go past the due date, clinicians typically offer monitoring and discuss induction around 41–42 weeks, depending on local guidance and individual circumstances. The calculator's job is to give you the starting estimate and the milestones; the decisions around it belong with your maternity care team.