How to Tell the Exact Date of Conception in Pregnancy

To calculate your exact date of conception you'll need to know the start date of your last menstrual period, how many days in your menstrual cycle, or when you ovulated. If you don't have that information, you can work backwards from your due date, with the help of an ultrasound and your health care provider. It's important to remember that 80 percent of babies are born within 2 weeks of their due dates, but less than 5 percent of babies are born on their precise due dates.

Calculate your conception date by counting backwards from your due date. Credit: payphoto/iStock/Getty Images

Step 1

Subtract 266 days, which is 38 weeks, from your due date to know the date of conception. Your health care provider can help you determine your due date by using ultrasound pictures and charting the growth of your baby.

Step 2

Add 14 days to the start date of your last menstrual period. This should be your ovulation date. Conception occurs within 24 hours of ovulation. If your cycle is longer or shorter than 28 days, this calculation won't be entirely accurate.

Step 3

Track your ovulation for a precise conception date. One way to track ovulation is by taking a basal temperature reading using a basal thermometer, which is more sensitive than an ordinary thermometer. Take an oral temperature reading as soon as you wake up, before getting out of bed. Your temperature will be slightly elevated during ovulation.

Step 4

Use an at home ovulation prediction kit, available at most drug stores. Much like a pregnancy test, it detects a hormone surge in urine. During ovulation there is a surge of luteinizing hormone; this is the most precise way to detect ovulation at home. Conception occurs within 24 hours of ovulation.

Things You'll Need

  • Calendar

  • Ultrasound

  • Basal thermometer

  • Ovulation prediction kit


Look for changes in cervical mucus as a sign of impending ovulation.


Your most fertile time is the entire week of ovulation and having sex up to 5 days before ovulation can result in pregnancy.

Load Comments

Copyright © 2019 Leaf Group Ltd. Use of this web site constitutes acceptance of the LIVESTRONG.COM Terms of Use , Privacy Policy and Copyright Policy . The material appearing on LIVESTRONG.COM is for educational use only. It should not be used as a substitute for professional medical advice, diagnosis or treatment. LIVESTRONG is a registered trademark of the LIVESTRONG Foundation. The LIVESTRONG Foundation and LIVESTRONG.COM do not endorse any of the products or services that are advertised on the web site. Moreover, we do not select every advertiser or advertisement that appears on the web site-many of the advertisements are served by third party advertising companies.