Conception is counted back 266 days from the due date
A full-term pregnancy runs 280 days (40 weeks) from the first day of the last menstrual period, and conception happens about 14 days after that period. Subtract the difference and conception lands roughly 266 days before the due date. The result is an approximation, not a fixed day.
Due date, conception and last period are three different markers
- Last menstrual period (LMP). The first day of the last period, and the point all pregnancy dating counts from. It sits 280 days before the due date.
- Conception. When the egg is fertilized, about 14 days after the LMP. On the medical clock this is already "2 weeks pregnant."
- Due date. The estimated day of delivery, 40 weeks after the LMP. Only about 1 in 20 babies actually arrive on it; roughly 9 in 10 arrive within two weeks of it.
Conception is a window, not a single day
Sperm can survive up to five days and the egg lives about a day after release, so intercourse over a span of several days can lead to the same conception. Treat the calculated date as the middle of a fertile window roughly five days wide on each side.
- Irregular cycles shift it. With a longer or shorter cycle than 28 days, ovulation moves, so the true conception date can be a week or more off the average.
- Ultrasound tightens it. A first-trimester dating scan is accurate to about three days, so a due date from that scan gives a much narrower conception estimate than one from the last period alone.
Common questions
How accurate is a conception date worked back from a due date?
It is an estimate, not a fact. The real conception could fall within about five days on either side of the calculated date, because ovulation timing and cycle length vary. A due date set by a first-trimester ultrasound narrows the range; one set only from the last period widens it.
Why is conception 266 days before the due date and not 280?
Medicine dates pregnancy from the first day of the last menstrual period, which is about 14 days before conception. The full term is 280 days from that period, so conception sits roughly 266 days before the due date.
Can I get a conception date from my own birthday?
Roughly. Subtract about 266 days from your birth date for an approximate conception date. The margin is wider here, plus or minus a week or more, because the exact due date and the mother's cycle length are unknown.
