Determining the Date of Conception Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception.
For women with a menstrual cycle of average length, that day is usually about two weeks before conception, which explains why pregnancies are said to last 40 weeks.
This method doesn't take into account how long your menstrual cycle actually is or when you think you might have conceived.
If you do happen to know precisely when you conceived – perhaps you were tracking your ovulation and you know on which day your egg must have been fertilized – we give you the option of calculating your due date based on your conception date. Your healthcare provider might revise your due date if she has reason to believe it's off target.
She'll measure your baby during an ultrasound exam, and if your baby is much bigger or smaller than would be reasonable given your due date, she'll provide you with a new due date.(For a fun read, see our stories from parents who know when and where they did the deed that resulted in their child.)Of course, a due date calculation is always approximate, whether it's from our tool or from your doctor or midwife. You're just as likely to go into labor any day during the two weeks before or after.
It's possible that the sperm hung out in your fallopian tube for a few days until your egg came along, at which point it fertilized the egg and your baby was conceived.
So without knowing the day of conception, how does anyone determine a due date? The method typically used by healthcare providers is simply to count from the first day of your last period.
The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.
"Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" .
Thus, their conception date is merely an estimate based on the first day of their last period.