Unless you got pregnant through a procedure such as in vitro fertilization or artificial insemination, the only way to know with 100 percent certainty the date you conceived a child is if you had sexual intercourse only once during your fertile period. In that case, the conception date is simply the date you had sex. Otherwise, the best you can do is an estimate. Doctors derive that estimate in much the same way they calculate your baby's due date.
Last Menstrual Cycle
Identify when your last menstrual cycle began. All dates and time frames related to pregnancy--such as the baby's due date, the gestational age and the conception date--are pegged to your last menstrual period. When you go in for your first pregnancy appointment with your OB/GYN, the doctor or a nurse will ask you for the first day of that period--that is, the day that bleeding started. To calculate your due date, doctors simply count off 40 weeks starting from that day. So if it's later in your pregnancy and you've forgotten the start day of your last period, just grab a calendar, find your due date and count back 40 weeks.
Conception Date
From the start date of your last period, add two weeks. Every woman has her own cycle, and ovulation--the release of an egg cell from an ovary, which makes it possible to get pregnant--occurs at different times for different women. Generally, though, ovulation occurs anywhere from about 11 to 21 days from the start date of your cycle, says the American Pregnancy Association. Once an ovary releases an egg cell, the egg must be fertilized within about 24 hours or it will die. Since sperm can survive in the body for 2 to 3 days, that means the "window" for getting pregnant extends from a couple days before ovulation to a day afterward, according to Baby Center. Some women have a more regular cycle than others, and some are even able to tell when they are ovulating. If this is the case, you may be able to narrow down the conception date further. If you've been keeping track of when you've had sex, this may allow you to pinpoint the exact date.
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