Naegele's rule and the adjustment for cycle length
The calculator uses Naegele's rule: the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period (LMP). The same result comes from adding seven days to the LMP and counting forward nine months. The 280 days assume a 28-day cycle with ovulation on day 14, so for another cycle length the tool shifts the date by the difference. A 32-day cycle adds 4 days, a 26-day cycle takes off 2.
Estimated conception is LMP + 14 days, shifted by the same cycle adjustment. "How far along" is the number of whole days from the LMP to today, shown as weeks and days. The trimester label switches at 13 and 27 weeks, so weeks 0–12 show first, 13–26 second, and 27 onwards third.
Worked examples
| LMP | Cycle | Due date | Conception |
|---|---|---|---|
| 1 Jan 2026 | 28 days | 8 Oct 2026 | 15 Jan 2026 |
| 1 Jan 2026 | 30 days | 10 Oct 2026 | 17 Jan 2026 |
| 10 Mar 2026 | 28 days | 15 Dec 2026 | 24 Mar 2026 |
How much to trust the date
- It is a midpoint. Only about 4% of births land on the estimated date, and a normal range stretches a couple of weeks either side.
- Dating from the LMP assumes you remember it. An uncertain or irregular LMP makes the estimate less reliable, and a dating ultrasound usually takes over when the two differ. Midwives and doctors set the official date.
- Gestational age counts from the LMP. That is why a pregnancy is "4 weeks" at about the time of a missed period, even though conception was only two weeks earlier.
- Cycle length field. It accepts 20 to 45 days. Use your average cycle, not the longest or shortest one.
- Not for IVF. Embryo transfers are dated from the transfer or egg retrieval date by the clinic, which this tool does not model.
This page gives an estimate only and is not medical advice. If you are tracking fertile days before conception, use the ovulation calculator, which covers the same cycle logic in the other direction. To see how many days remain until the date, use the date difference calculator.