Due on January 20? Your Most Likely Conception Date

Due dates & conception. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

Wondering when your January 20 baby was conceived? Using the standard 266-day rule, the answer is most likely April 29 of the previous year, give or take a few days.

Key dates for a January 20 due date

How the conception date is calculated

Due dates are normally counted as 280 days (40 weeks) from the first day of the last menstrual period. In a typical 28-day cycle, ovulation and conception happen about two weeks after that period starts, so conception sits roughly 266 days before the due date. Subtracting 266 days from January 20 gives April 29.

Because sperm can live for up to five days, the act of sex and the moment of conception aren't always the same day. Fertilisation happens close to ovulation, but the sex that led to it may have been days earlier.

How certain is this date?

Remember that a due date is only the midpoint of a range. Ovulation timing varies from cycle to cycle, so the true conception day can sit a few days away from the estimate.

If your cycles are usually longer than 28 days, conception probably happened a little later than April 29; with shorter cycles, a little earlier. A cycle that is 32 days long, for example, shifts the likely date about four days later.

Check your own dates

Enter your exact due date in the conception date calculator to see these results with weekdays and the correct year, or start from your last period with the last-period conception calculator.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.