Conception Date Calculator

How it is calculated

The tool works backwards from the estimated due date: conception = due date − 266 days. 266 days (38 weeks) is the typical time from conception to birth, which is two weeks shorter than the 40 weeks counted from the last menstrual period. It then adds 6 and 12 days to the conception date to show the window in which implantation is generally expected, and gives the weekday of the conception date. The date must be a valid calendar date in year-month-day form. The result is an estimate, because ovulation timing and pregnancy length differ between people, and the actual conception can fall some days either side.

Common mistakes

• Entering the date of the last menstrual period instead of the due date; the tool expects the due date.
• Treating the result as an exact day: it is a mid-point estimate, and cycles that are not 28 days shift it.
• Mixing up conception with the start of the pregnancy count, which doctors date from the last period, about two weeks earlier.
• Using a due date from an early ultrasound and a different one from the period date without deciding which one your doctor uses.
• Expecting the weekday and the dates to depend on a time zone; they are plain calendar dates.