How to Determine Conception Date Late in Pregnancy

If you're pregnant, knowing the conception date of your child has medical and sentimental value. However, it can be tricky to pinpoint the exact date of conception, and this calculation becomes more difficult further along in the pregnancy. Fortunately, you can take a few steps to help approximate the date of conception.

Instructions

• 1

  Calculate the first day of your last menstrual period. This is the date that physicians use to approximate the age of the fetus. This gestational age is intentionally calculated incorrectly. Since the date of conception is difficult to pinpoint, obstetricians use the first day of your last menstruation as a fixed date. In fact, this cannot be the actual date of conception (conception is highly unlikely to occur during menstruation). However, you can use it as a tool to approximate the date of conception.

• 2

  Add 14 days to this date. This will generate a rough estimate of the date of conception. However, understand that this is only a ballpark figure. Sperm can live for seven days inside a woman's reproductive tract, so it is impossible to pinpoint the exact date of conception.

• 3

  Look at the ultrasound to estimate the gestational age of the fetus. Your obstetrician or ultrasound technician will know the expected growth levels of a fetus and can estimate its age. For instance, at 22 weeks the lips of the fetus become distinct and pronounced; this can be used as a marker to calculate gestational age. However, this technique becomes more difficult late in the pregnancy; the finer details of fetal development, such as fingernails and eyelashes, may develop at varying times in the third trimester.