When did I conceive?

Medicine dates pregnancy two ways that always sit exactly two weeks apart: 280 days from the first day of the last menstrual period, or 266 days from conception (ACOG Committee Opinion 700). So this tool simply runs the rule in reverse — conception is about 266 days before your estimated due date. Because ovulation can't be pinned to a single day even in a well-dated pregnancy, we show a window of about five days on either side rather than one date. If your due date came from an early ultrasound, this back-calculation is about as accurate as conception dating gets.

Because biology doesn't allow a single day, no matter how good the math is. The exact day of ovulation is always an estimate — it varies between cycles even in people with regular periods. On top of that, sperm can survive in the reproductive tract for up to about five days, while the egg can only be fertilized for roughly 12 to 24 hours after ovulation. Put together, conception could have followed sex on any of about six days, and the ovulation estimate itself carries a few days of uncertainty. Any tool that hands you one definitive date is overselling what the math can do.

No — and no calculator can. Because sperm survive up to five days, sex on any day in roughly a six-day span ending at ovulation could have led to conception, and the ovulation estimate itself has a window of several days. If two encounters fall inside or near that range, date math cannot separate them, full stop. The only reliable way to establish paternity is DNA testing — prenatal options exist, and testing after birth is straightforward. If this question matters for you, it's worth discussing with your provider rather than relying on any calendar estimate.

Because pregnancy is counted from the first day of your last period, not from conception — a convention that predates ultrasound and is still used everywhere in medicine. In a textbook 28-day cycle, ovulation happens around day 14, so the egg is fertilized about two weeks after the period that 'starts' the pregnancy count. That's also why you're already called four weeks pregnant when your period is only a few days late: the count includes the two weeks before conception even happened. In longer or shorter cycles, ovulation shifts with cycle length, which is why this tool asks for yours.

It depends entirely on the input. A due date set by a first-trimester ultrasound is the most accurate anchor there is — ACOG calls the early scan the gold standard for dating a pregnancy, and it overrides period-based dates when they disagree by more than a few days. Back-calculating from that kind of due date gives a solid window. Period-based math is looser: it assumes you ovulated exactly (cycle length − 14) days in, and real ovulation wanders a few days even in regular cycles — more if yours are irregular. Whatever the input, treat the result as a well-grounded estimate to discuss with your provider, never a fact.

Completely. This calculator runs entirely in your browser. The dates you enter are never sent to a server, stored, or shared — close the tab and everything is gone. We treat reproductive data as sensitive by default, so there is nothing here to subpoena, leak, or sell: the tool has no record that you ever used it. It's an educational estimate based on published ACOG dating methods, not medical advice, a diagnosis, or a legal determination of anything.