Add 266 days to the ovulation date, or add 266 days to the conception date. Both estimates use the same count because fertilization is usually within about a day of ovulation. The result is a planning date, not a birth appointment and not a diagnosis.
This count is not for IVF or embryo-transfer dating. Those pregnancies use the transfer date and the age of the embryo, which this calculator does not model.
Worked example
Ovulation on 14 March 2026 plus 266 days is 5 December 2026.
Conception on 15 March 2026 plus 266 days is 6 December 2026.
The one-day gap in the starting date becomes a one-day gap in the estimate. If you only know that conception was “around” ovulation, use the ovulation date and treat a day either way as noise. The due date by ovulation calculator accepts either date and labels it.
Why 266 days
A 28-day cycle is counted as 280 days (40 weeks) from the first day of the last period. Ovulation in that model is 14 days after the period starts, and 280 − 14 = 266. 266 days is 38 weeks.
If the cycle is not 28 days, the period method shifts by the difference. A known ovulation date already includes that shift, so you do not also add cycle days on top of the 266.
When the two methods disagree
The pregnancy due date calculator assumes ovulation on day 14 of the cycle, adjusted when you enter a different cycle length. If you ovulated on day 18, the period estimate is about four days earlier than the ovulation estimate. The ovulation date is the more direct of the two when you actually know it.
An early ultrasound replaces both numbers when they disagree. How a due date is calculated walks through the period formula. Ovulation versus conception is the timing difference. How many days a pregnancy is puts 266 and 280 side by side.