Conception Date Calculator

Working out the date of conception is really an exercise in working backwards. Sperm can survive up to five days in the reproductive tract, and an egg lives for roughly twenty-four hours after it is released. Conception therefore happens on the day of ovulation, or within a few days of it if intercourse took place earlier and sperm were already waiting.

The most reliable anchor is ovulation itself, and ovulation is best estimated from the other end of the cycle. The luteal phase — the stretch from ovulation to the first day of the next period — is remarkably stable at about fourteen days, while the first half of the cycle is the part that stretches and shrinks. So ovulation typically falls fourteen days before your next period was due. With a 28-day cycle that is day 14; with a 32-day cycle it is day 18.

If you already have a due date from a scan, you can run the arithmetic in reverse: subtract 266 days, or about 38 weeks, to reach the likely conception date. That is the convention clinicians use, and it is more accurate than counting from your period when you have a dating scan.

Two honest limits. First, unless you were tracking basal body temperature, cervical mucus or ovulation tests, this is an estimate with a window of several days — often enough to be interesting but not enough to settle a question that matters. Second, conception dates matter legally and emotionally in some situations, and a calculator cannot resolve those. If precise timing is important to you, ask your doctor or midwife what the scan suggests, and treat the number here as a good-natured estimate rather than evidence.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.