Pregnancy Due Date Calculator
Enter the first day of your last period and your average cycle length to estimate your due date, how many weeks pregnant you are today, and the approximate date range of each trimester.
| Today, you are | — |
| First trimester (weeks 1–12) | — |
| Second trimester (weeks 13–27) | — |
| Third trimester (week 28 onward) | — |
This is a generic calendar estimate, not a diagnosis or a substitute for antenatal care. It assumes a textbook 280-day pregnancy, adjusted only for your cycle length — the NHS says a normal pregnancy actually lasts anywhere from 37 to 42 weeks, so an on-the-ground delivery date can sit weeks either side of this figure and still be completely normal. Your midwife or doctor will confirm your dates, and adjust them if needed, using an ultrasound scan: NCBI's clinical reference on gestational age puts calendar methods like this one at roughly ±3 weeks of accuracy, against about ±5–7 days for a first-trimester scan. Nothing you type leaves your browser.
How the estimate is built
Step 1 — the base estimate (Naegele's rule). Named after the German obstetrician Franz Naegele, the rule adds 280 days — 40 weeks, or 9 months and 7 days — to the first day of your last period. It assumes ovulation on day 14 of a 28-day cycle and a fixed length of pregnancy from conception to birth.
Step 2 — the cycle-length adjustment. A longer or shorter average cycle shifts ovulation by roughly the same number of days, so this calculator adds (or subtracts) the difference: due date = last period + 280 days + (cycle length − 28). This is a simplified version of the correction clinical references describe alongside Naegele's rule for cycles that aren't exactly 28 days.
Step 3 — weeks pregnant and trimesters. "How many weeks pregnant" is conventionally counted as time elapsed since the first day of your last period, regardless of your cycle length — which is why that figure, and the trimester ranges below it, aren't cycle-adjusted the way the due date is.
- How is my due date actually calculated?
- This calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period, then adjusts by the difference between your average cycle length and the textbook 28 days. The rule assumes ovulation on day 14 of a 28-day cycle and a fixed gestation length from conception — real pregnancies vary around both of those assumptions, which is one reason a "due date" is an estimate rather than a booking.
- Why might my midwife or sonographer give me a different date?
- Because they're using a more accurate method. NCBI's clinical reference on gestational age assessment notes that calendar methods like this one carry a margin of error of roughly ±3 weeks, while measuring the embryo or fetus by ultrasound in the first trimester (crown-rump length) is accurate to within about 5 to 7 days. The NHS says the 12-week dating scan offered as part of antenatal care gives a more accurate estimate of how many weeks pregnant you are than working it out from your last period — so if the two disagree, your antenatal team will normally go with the scan, not this page.
- Is it normal to deliver before or after this date?
- Yes. The NHS describes a normal pregnancy as lasting anywhere from 37 to 42 weeks counted from the first day of your last period, so a due date marks the middle of a multi-week window rather than a deadline. If a pregnancy continues past 42 weeks, a midwife will usually discuss the options, including induction of labour, with you directly.
FAQ
Sources
Figures used: Naegele's rule (last period + 280 days / 40 weeks, assuming ovulation on day 14 of a 28-day cycle); the cycle-length adjustment (± the difference between your cycle length and 28 days) is this calculator's simplified version of the correction described alongside Naegele's rule in the clinical reference below; normal pregnancy length 37–42 weeks; calendar-method dating accuracy of roughly ±3 weeks versus ±5–7 days for first-trimester ultrasound (crown-rump length) dating. The trimester ranges shown (weeks 1–12 / 13–27 / 28+) match the NHS's own second- and third-trimester boundaries; the NHS's week-by-week guide starts the first trimester at week 4 rather than week 1, since pregnancy usually isn't confirmed before then — this page starts counting at week 1 (the last-period date) so the three ranges run back-to-back with no gap. Estimate only, not a diagnosis.
- NHS — Pregnancy due date calculator (due date from the first day of your last period; normal pregnancy lasts 37 to 42 weeks; the 12-week scan gives a more accurate estimate of how many weeks pregnant you are)
- NHS — Week-by-week guide to pregnancy (first trimester weeks 4–12, second trimester weeks 13–27, third trimester weeks 28–41)
- NHS — Periods (menstrual cycle around 28 days typical; normal range 21 to 35 days)
- NCBI Bookshelf (StatPearls) — Estimated Date of Delivery (Naegele's rule formula and its 28-day / day-14-ovulation assumption; calendar dating accurate to roughly ±3 weeks versus ±5–7 days for first-trimester ultrasound crown-rump length)
- NCBI Bookshelf (StatPearls) — Gestational Age Assessment (crown-rump length is the most accurate measurement for first-trimester dating; a later scan shouldn't override an already-established first-trimester estimate)