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A conception calculator estimates when conception likely occurred. From a due date it counts back 266 days (38 weeks), because fertilization happens about two weeks after the last period on a standard 28-day cycle and pregnancy runs 280 days from that period. From a last-period date it counts forward to ovulation (cycle length minus 14 days). Because sperm survive several days, a likely conception range is shown around the central date.

Working from a due date, subtract 266 days: a due date of, say, 40 weeks out places conception 38 weeks earlier, with the last period roughly two weeks before that (due date minus 280 days). Working from your last period, it adds cycle length minus 14 to find ovulation — day 14 on a 28-day cycle, day 16 on a 30-day cycle. It then shows today’s gestational age, measured from the last period rather than conception.

Use it to trace back roughly when a pregnancy began, either out of curiosity or to line up dates with a due date. The several-day range reflects that sperm can survive up to about five days, so conception may follow intercourse by a few days. Actual ovulation varies, so an early dating ultrasound is the most reliable check; this tool is for information, not medical advice.