Conception Date Calculator

About the Conception Date Calculator

Work backwards to the estimated date of conception from whatever you know: the due date on your scan report, a baby’s date of birth and how many weeks of pregnancy they were born at, the first day of the last period and your usual cycle length, or the date of an IVF embryo transfer. You also get the likely conception window — the 5 days before ovulation and the day of ovulation, when intercourse could have led to the pregnancy — plus the due date and how far along the pregnancy is today.

The estimate uses the same conventions doctors use to date a pregnancy, so it agrees with your antenatal notes. It is still an estimate: ovulation does not happen on the same day for everyone, and babies are not born exactly at 40 weeks. The page explains how big that uncertainty is for each method, and why a conception date can never show who the father is.

How the dates are worked out

Pregnancy is dated from the first day of the last menstrual period (LMP). By convention the due date is 280 days (40 weeks) after it, with ovulation and conception on day 14 of a 28-day cycle — so conception is about 266 days before the due date.

• From a due date: conception ≈ due date − 266 days.
• From a birth: the equivalent LMP is the birth date minus the gestational age at birth (39 weeks + 0 days = 273 days); conception ≈ that date + 14 days.
• From the last period: ovulation is taken as 14 days before the next period would have started, so conception ≈ LMP + cycle length − 14. A 32-day cycle puts it on day 18; the due date moves 4 days later too.
• From an IVF transfer: conception = transfer date − the embryo’s age (3, 5 or 6 days). ACOG dates IVF pregnancies this way: the due date is 261 days after a day-5 transfer and 263 days after a day-3 transfer.
• The conception window: in a study that tracked ovulation with daily hormone tests, pregnancies came only from intercourse in a 6-day window ending on the day of ovulation; the chance of conceiving rose from about 10% for intercourse 5 days before ovulation to 33% on the day itself. Sperm can survive that long, so the intercourse that led to a pregnancy can be up to 5 days before conception.

How uncertain is it?

• Ovulation moves around. In a study of 221 women, only about 30% had their whole fertile window between days 10 and 17 of the cycle; many ovulated earlier or later, even with regular cycles. A date worked out from the last period carries that uncertainty.
• An early scan is the best anchor. ACOG changes a due date based on the last period only when a first-trimester ultrasound differs by more than 5–7 days, so a due date from an early scan pins conception down to about a week.
• Birth dates are the least precise. When conception dates were known exactly, the time from ovulation to birth had a median of 268 days and still varied by 37 days among full-term births. Working back from a birthday therefore gives a range of weeks, not a day — unless the gestational age at birth was set by an early scan.
• After IVF the embryo’s age is known, so the dating date is exact.

A conception date cannot establish paternity

Because sperm can survive for up to 5 days, ovulation varies from cycle to cycle and every method above is uncertain by a week or more, intercourse on days far apart can fit the same estimate. A calculator cannot tell who the father is, and neither can a due date or a scan. Only a DNA test can establish paternity.

Sources

• ACOG Committee Opinion No. 700, Methods for estimating the due date
• Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med
• Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ
• Jukic AM et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod

Limitations

• All dates except the IVF date are estimates; allow a week or two either side, more when working back from a birth.
• The last-period method assumes ovulation 14 days before the next period. If you tracked ovulation (tests, temperature), that date is more reliable than any calculation.
• It cannot date a pregnancy that is not progressing normally, and it does not replace dating by a clinician or an ultrasound scan.
• It cannot establish paternity.

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