Most people try to work out their conception date by counting back from the due date and landing on a weekend they remember. It rarely gives the right answer, and the reason is worth knowing.
Pregnancy is dated from the first day of your last menstrual period — a point at which you were not pregnant at all. Clinicians add 280 days to that date to get a due date, a method known as Naegele's rule. Ovulation, and therefore conception, usually happens around two weeks later. So the moment you are told you are four weeks pregnant, the embryo is closer to two weeks old.
The second complication is sperm survival. Sperm can live in the reproductive tract for up to five days in fertile cervical mucus, while an egg is viable for only 12 to 24 hours after release. If intercourse happened on a Monday and ovulation on the Thursday, conception happened Thursday. The day you remember and the day it happened can be several days apart.
Cycle length changes everything too. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles run 35 days, you are likely ovulating around day 21 — a full week later than the formula assumes. The reliable principle is that ovulation sits roughly 14 days before your next period, not 14 days after your last one. So a better estimate is cycle length minus 14.
If you want to see how much your own cycle length shifts the result, this conception date calculator takes your real cycle length instead of assuming 28 days. The difference is usually larger than people expect, and it moves the due date by the same amount.
One last point: if you have had an early dating scan, use that date over any calculation. Between 8 and 13 weeks, embryos grow at a remarkably consistent rate, which makes crown-rump length accurate to within about five to seven days. After 22 weeks that accuracy disappears, which is why the early scan is the one that sets the date for the rest of the pregnancy.
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