Due Date by Conception Date: 266 Days

If you tracked ovulation and know roughly when conception happened, you can skip the period entirely: conception plus 266 days. Enter the fertilization date below.

Why 266 rather than 280

A term pregnancy is conventionally 280 days from the first day of the last period. But the period starts before anything is conceived. In a textbook 28-day cycle, StatPearls places ovulation “around day 14,” with the follicular phase running from day 1 to day 14 and the luteal phase from day 14 to day 28.

Subtract those two weeks and the count from fertilization is 280 minus 14 = 266 days, or 38 weeks. Same due date, different starting line. If you want the number the other way around — your period in, the conception day out — that is the conception date calculator.

Worked example

Estimated conception on March 15:

• March 15 plus 266 days gives a due date of December 6.
• The equivalent last period would have started around March 1, and March 1 plus 280 days also gives December 6. The two routes agree by construction.

Conception is a window, not a day

This is where the method leaks precision. The Office on Women’s Health puts numbers on it: sperm can live 3 to 5 days in the reproductive tract, while the egg lives only 12 to 24 hours after ovulation. So intercourse on Monday can produce fertilization on Thursday. You know the act; you rarely know the moment.

The cycle itself is looser than the textbook too. Across 3,324 U.S. cycles, ovulation was estimated on cycle day 14 in only 8.7% of them, and a 14-day luteal phase appeared in 9.7% — the idealized cycle turns out to be the minority case. So use this page when your ovulation data is real, from tests or basal temperature, and treat the output as a midpoint.

When something better exists, use it

ACOG ranks first-trimester ultrasound above every calendar method: crown-rump length dates a pregnancy to within about 5 to 7 days through 13 weeks and 6 days. If your calendar date and an early scan disagree by more than the thresholds on due date by ultrasound, the scan is the one that goes in the chart.

The exception is IVF, where the conception date is not estimated but recorded. That case has its own arithmetic on the IVF due date calculator.

This content is informational and does not replace prenatal care. Dating a pregnancy is done in the clinic, with the ultrasound in hand.