Pregnancy Due Date Calculator
Work out your estimated due date from the first day of your last period or your conception date — and see how many weeks and days pregnant you are today.
From the first day of your last period
Today's date is taken from your device clock. Estimates only — your midwife or doctor will use the dating scan as the official due date once it's available.
Formula
Two short formulas — pick the tab for what you have:
- From LMP:
due date = LMP + 280 days + (cycle length − 28). The cycle adjustment shifts ovulation, and so the due date, by the same number of days. - From conception:
due date = conception + 266 days.
Trimesters use the standard clinical boundaries: trimester 1 ends at 14 weeks 0 days, trimester 2 at 28 weeks 0 days, and trimester 3 runs from there to the due date.
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Frequently asked questions
How is a due date calculated?▾
The standard method is Naegele's rule: take the first day of your last menstrual period (LMP) and add 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14, so the actual gestation from conception is 266 days. The NHS uses this same calculation when no scan dating is available, then refines it with the dating scan at 8–14 weeks if your cycle is irregular or you're unsure of your LMP.
What if my cycle isn't 28 days?▾
Adjust the cycle length input — every day longer than 28 pushes the estimated due date one day later (because ovulation, and so conception, was a day later). The calculator uses (cycle length − 28) as the adjustment. For very irregular cycles a dating scan is more accurate; the NHS offers one to all pregnancies between 8 and 14 weeks and that scan's measurement takes priority over the LMP date.
Will I actually give birth on the due date?▾
Probably not exactly. Only about 4–5% of UK babies arrive on their estimated due date; most arrive within a window roughly 37–42 weeks (any time from three weeks before to two weeks after). The NHS classes 37–42 weeks as full term; if you reach 41 weeks you'll usually be offered a membrane sweep, and induction is typically discussed around 41+3 to 42 weeks.
How do trimesters work?▾
Trimester 1 covers weeks 0–13 (LMP to end of week 13), trimester 2 weeks 14–27, and trimester 3 from week 28 to delivery. The boundaries are conventional rather than biological — they roughly correspond to the early-pregnancy phase, the more settled middle phase, and the late phase. The dating scan and the 12-week and 20-week NHS appointments are landmarks of trimesters 1 and 2.
When should I tell the NHS I'm pregnant?▾
Contact your GP surgery or self-refer to your local maternity service as soon as you know — ideally before 10 weeks. The first booking appointment with a midwife is normally at 8–10 weeks, where they take your history and arrange the dating scan. The earlier you book in, the more time there is to arrange screening tests that have a window (combined screening for Down's syndrome, for example, is 11+0 to 14+1 weeks).