How the date is estimated
Pregnancy is counted as 40 weeks (280 days) from the first day of the last period, which assumes ovulation on day 14. Other starting points are converted to the same clock:
| Based on | Due date |
|---|---|
| Last period | + 280 days, adjusted for cycles longer or shorter than 28 days |
| Conception | + 266 days |
| IVF transfer | + 263 days (day-3 embryo) or + 261 days (day-5) |
| Ultrasound | Scan date + (280 − age of pregnancy at the scan) |
Common questions
Which method is most accurate?
An early ultrasound (before 14 weeks) is generally considered the most accurate, since it directly measures the fetus rather than assuming a standard cycle. LMP-based dating assumes ovulation on day 14, which doesn't hold for everyone with irregular cycles.
Why 266 days from conception but 280 from the last period?
Both land on the same due date for a 28-day cycle — conception typically happens about 14 days after the period starts, so 266 + 14 = 280. The calculator adjusts for your actual cycle length rather than assuming exactly 28 days.
An estimate for planning only. Your doctor or midwife will confirm the date, usually from an early scan.