Pregnancy Due Date Calculator

How to use the pregnancy due date calculator

Choose whichever date you know most confidently — the first day of your last menstrual period, an estimated conception date, or an IVF embryo transfer date — and enter it. If you're using your last period date, also enter your average cycle length, since this affects when ovulation (and therefore conception) likely happened. The calculator then estimates your due date, shows your current week and day of pregnancy, tells you which trimester you're in, and lists key NHS antenatal milestones with estimated dates.

This tool is designed to give you a helpful estimate to plan around — for confirming exact dates and monitoring your pregnancy, always follow the guidance of your midwife, GP or hospital, who will use ultrasound measurements alongside your dates.

How the due date is calculated

The standard method used across the NHS and by most pregnancy calculators is known as Naegele's rule: your due date is estimated as 280 days (40 weeks) after the first day of your last menstrual period, assuming a typical 28-day cycle with ovulation on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts the due date by the difference — a 32-day cycle, for example, pushes the estimated due date back by four days, since ovulation likely happened later.

If you know your conception date directly, the calculator adds 266 days (38 weeks) — the average length of pregnancy measured from conception rather than last period. For IVF pregnancies, the due date is calculated from the known age of the embryo at transfer: 261 days after a day-5 blastocyst transfer, or 263 days after a day-3 transfer, since the embryo's exact age is known precisely rather than estimated.

Worked example

If the first day of your last period was 1 May and you have a typical 28-day cycle, your estimated due date would fall around 5 February the following year — 280 days later. By early August, roughly 13 weeks on from 1 May, you'd be entering your second trimester, with your dating scan likely to have already taken place around weeks 11–14.

If your cycles typically run longer, say 32 days, the calculator would shift your due date about 4 days later than the standard calculation, to around 9 February, reflecting the likelihood that ovulation happened a few days later than the 28-day-cycle assumption.

Trimesters and key NHS milestones

Pregnancy is typically divided into three trimesters: the first trimester runs from conception to around week 12, the second from week 13 to around week 27, and the third from week 28 until birth. Along the way, the NHS offers a set of standard antenatal appointments and scans: a booking appointment around weeks 8–10 to confirm your pregnancy and discuss your care, a dating (12-week) scan between weeks 11–14, and an anomaly (20-week) scan between weeks 18–21. This calculator estimates the timing of each based on your dates, so you know roughly when to expect your midwife or hospital to be in touch.

Why your due date might change

It's common and completely normal for a due date to be adjusted slightly after your 12-week dating scan, once your baby's measurements are compared against standard growth charts — this is generally considered more accurate than a date based on your last period alone, particularly if your cycles are irregular. If your due date changes after a scan, your care team will use the updated date going forward.

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