How to Calculate Time of Conception Based on Date of Birth

Written by phillip woolgar
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How to Calculate Time of Conception Based on Date of Birth
Most pregnancies last nine months. (pregnant image by Cora Reed from

You can estimate the date of conception based on a child's birth date. In just a few steps, you can more easily pinpoint the estimated day, though it is only an estimate. It is impossible to find an exact conception day because the duration of the pregnancy varies from woman to woman and from pregnancy to pregnancy. Most women stay pregnant for approximately nine months.

Skill level:

Things you need

  • Calendar

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  1. 1

    Track back 266 days from the baby's date of birth on a calendar. The average period of gestation is 280 days, though 280 starts from the time of the woman's last menstrual cycle. This includes two weeks before conception. Subtracting the 14 days from 280 gives you the 266 figure.

  2. 2

    Use that date to create a range when the conception could have occurred. Add a week ahead and a week behind the date you got when subtracting 266 from the birth date. This is the range when the conception took place.

  3. 3

    Try to recall the days in this period when sexual intercourse happened. One of those days would be the actual conception date. Sperm, however, can survive up to five days after sex, so the actual conception could have occurred up to five days after the date of sexual intercourse.

Tips and warnings

  • To more accurately pinpoint the day of conception, you can measure the foetus during early pregnancy via an ultrasound.
  • An accurate way to determine the period of time in which conception probably occurred is to calculate the first day of the last menstrual period and add 14 days, according to the American Pregnancy Association.
  • Any calcuation of the conception date might not be accurate, even with the ultrasound technique. Other variables, such as a baby that grows quicker or slower than average or a woman with irregular periods, need to be considered. Babies also have various gestational periods, which could also skew results.

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