Due date is commonly estimated from the first day of the last menstrual period using a 280-day or 40-week model. It works best when cycles are relatively regular, but it is still an estimate.
Due date is commonly estimated from the first day of the last menstrual period using a 280-day or 40-week model. It works best when cycles are relatively regular, but it is still an estimate.
The most common clinical 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 it is most accurate for regular cycles and carries a predictable bias for longer or shorter ones.
If your average cycle is 32 days you usually ovulate later, so the due date shifts about 4 days later than Naegele's result; a 25-day cycle shifts it roughly 3 days earlier. Counting from ovulation is cleaner: due date ≈ ovulation day + 266 days.
Between 8 and 13 weeks the fetal crown-rump length (CRL) varies little between pregnancies, so ultrasound dating is typically accurate to within ±5 days. If the ultrasound and LMP estimates differ by more than 5–7 days, clinicians usually re-date the pregnancy by the scan.
Fewer than 5% of babies arrive exactly on the due date. About 80% of single babies are born within three weeks of it (37–42 weeks), which is normal. Treating the due date as the centre of a window rather than a fixed day helps reduce anxiety.
Yes. If an early ultrasound differs noticeably from the LMP estimate, your clinician usually re-dates the pregnancy by the scan, and later visits use the new date.
You can get a rough estimate, but accuracy is limited. An early-pregnancy ultrasound is the most reliable option, so book that first scan early.
Often, yes. Term is 37–42 weeks, and going up to a week past the due date is usually normal, though your provider may increase monitoring or discuss induction.
Medical disclaimer: This guide is for general education and pregnancy date planning only and does not replace professional medical advice, diagnosis or treatment. For your individual situation, consult your doctor or midwife.
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Written and checked by the Pregnancy Tools editorial team · Last reviewed 2026-06-24 · About a 2-minute read · See About us