How to Figure the Day of Conception by Due Date

If you know your baby's due date, you can use that to estimate the day your child was conceived. However, it will only be an estimate; sperm can live inside the cervix for up to five days, and eggs can take 12 to 24 hours to shed during a regular menstrual period. Also, ultrasound estimates of your due date can be off if your baby is larger or smaller than average. Still, you can take a good guess as to when your baby was conceived.

Things You'll Need

• A calendar
• Internet access

Instructions

How to Figure the Date of Conception Based on Due Date

1. Due dates are estimated based on a typical 40-week gestational period. If you know the due date, calculate back 40 weeks. That is your estimated day of conception.

2. Figure out the average length of your cycle. Do you get your period every 28 days like clockwork or is it closer to 21 or 35 days? If your cycle is fairly regular, you can calculate the conception date by figuring out at which point in your cycle you were most fertile.

   The first day of your period is considered Day 1 of your cycle. Ovulation happens in the middle of your cycle. For a 28-day cycle, ovulation happens on Day 14, or about two weeks after the first day of your last period.

3. If you aren't sure about the length of your cycle but you do know the first day of your last period, you can use the Internet to access a conception date calculator, like the one at She Knows.com.

References

• Conception Calculator at She Knows.com
• American Pregnancy.org

Resources

• Read this Article in Spanish