According to the American Pregnancy Association, a baby's estimated conception date is between 11 to 21 days after the first day of the menstrual period in a woman who has regular periods. Calculating a conception date only provides a rough estimate, because different factors affect the ovulation time frame, such as irregular periods or menstrual cycles less than 21 days or longer than 35 days. Generally, only women who undergo certain fertility procedures, such as artificial insemination or in vitro fertilization, will know their exact dates of conception. Calculate your conception date by using the date of your last menstrual period, using your projected due date or by using an easy online calculator.
Calculating With Your Last Menstrual Period
Step 1
Determine the first day of your last menstrual period, and circle that date on a calendar.
Step 2
Add 11 days to the first day of your last menstrual period, and mark that date with an X.
Step 3
Add 21 days to the first day of your last menstrual period, and mark that date with an X also. The dates between the Xs are dates during which you could have conceived your baby. A typical woman ovulates on day 14 of her menstrual cycle, but even a woman with a regular period can ovulate as early as day 11 of her cycle or as late as day 21 of her cycle.
Calculating With Your Due Date
Step 1
Circle your estimated due date on a calendar.
Step 2
Subtract 266 days from the estimated due date.
Step 3
Adjust the year if needed. Your conception date should be within five days of this date.
Calculating With Online Conception Date or Due Date Calculator
Step 1
Enter your estimated due date or the first date of your last menstrual cycle in the appropriate field.
Step 2
Enter the average length of your menstrual cycle if required.
Step 3
Click "Enter" or "Calculate," or follow the on-screen prompts to calculate the estimated date of conception.
Tips and Warnings
- According to the Mayo Clinic, sperm can remain alive in the cervical mucus of a woman's vagina for three to five days, and conception can occur during this time.
