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.
When this calculator helps
This calculator gives you an estimated due date from the first day of your last menstrual period using Naegele's rule — the same method NHS midwives reach for when no scan dating is yet available. It adds 280 days, or 40 weeks, to that first day, which is the conventional length of a pregnancy measured from the start of your last period rather than from conception. It is the natural first step the moment you have a positive test and want a rough idea of when your baby might arrive.
Use it for planning rather than precision. Knowing roughly when you are due helps you think about when to tell work, when to start maternity leave, and when key NHS appointments are likely to fall. But it is genuinely an estimate: only about 4% of babies arrive on the exact date the rule produces, so treat the figure as the middle of a range, not a deadline circled on the calendar.
How to read your result
The date the calculator shows is your estimated due date — the point at which you would be 40 weeks pregnant counting from your last period. Around it sits the full-term window the NHS uses: any birth between 37 and 42 weeks is considered term, so a healthy arrival anywhere from roughly three weeks before your date to two weeks after is entirely normal.
The result also places you in a trimester. The first trimester runs from your last period to the end of week 13, the second from week 14 to week 27, and the third from week 28 until your baby arrives. These boundaries are conventions rather than biological switches, but they are a useful shorthand for which stage of the pregnancy — and which set of appointments and checks — you are currently in.
A worked example
Suppose the first day of your last period was 01/02/2026 and you have a typical 28-day cycle. Adding 280 days lands on an estimated due date of 08/11/2026. If today were 01/06/2026, that would put you at roughly 17 weeks pregnant — comfortably into the second trimester, with the 20-week anomaly scan still a few weeks ahead. Shift the start date a week earlier and the whole window slides a week earlier too, which is why getting that first date right matters.
Common mistakes to avoid
A few small misunderstandings can throw the estimate out by days or even weeks, so it is worth checking these before you rely on the number.
- Assuming a 28-day cycle when yours is consistently longer or shorter — every day your cycle runs over 28 pushes the due date a day later, so enter your real cycle length.
- Treating the due date as fixed; it is the centre of a 37–42 week window, not a guaranteed delivery day.
- Confusing gestational age (counted from your last period) with fetal age (counted from conception, about two weeks less) — pregnancy weeks are quoted in gestational age.
- Using a half-remembered date for your last period when an early scan would give a far more reliable starting point.
How this fits NHS antenatal care
In the UK your LMP-based estimate is only the starting point. Once you book in with a midwife — ideally before 10 weeks — you will be offered a dating scan between roughly 8 and 14 weeks. That scan measures the baby directly and, if it disagrees with your last-period date by more than about a week, the scan date becomes your official due date. A second anomaly scan at around 20 weeks checks the baby's development in more detail.
So if your confirmed due date from the dating scan differs from the figure here, follow the scan. This calculator is informational only and does not replace NHS antenatal care — always go with the date your midwife or doctor confirms, and raise any concerns about your pregnancy with them directly.
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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).