The short answer to the question, “Can I find out the exact day I got pregnant?” is, probably not. Determining the exact date of conception is difficult for several reasons.
First of all, you need to know that pregnancy occurs when an ovulated female egg and and male sperm meet and fertilization occurs. This usually takes place in a fallopian tube.
The time of ovulation, when an egg is released from the ovaries and into the fallopian tube, determines a woman’s fertility window and varies from woman to woman. Most women ovulate 14 days after the first day of their last menstrual cycle but some women release an egg anywhere from 11 to 21 days after the first day of their last period. An egg is viable, able to be fertilized, up to 24 hours after it has been released from the ovary.
Also, sperm can live in a woman’s body and fertilize an egg for up to seven days. This means there is a window of a week or so in which a woman can conceive after a sexual encounter.
Because the date of conception can be so hard to determine it has become standard practice to place a woman’s due date at 40 weeks after the first day of the woman’s last menstrual cycle. For women who don’t have regular periods, this can be a problem.
An ultrasound can more accurately determine the date of conception and the gestational age of the baby. In an ultrasound measurements are taken of the gestational sack and the crown to rump length of the fetus. These measurements are used to estimate the gestational age of the baby. The baby can begin to be measured between 5 and 6 weeks after the first day of a woman’s last period, but measurements are more accurate between 8 and 18 weeks after a woman’s last period.
Even with an ultrasound exam the exact date of conception can be hard to pin down because the baby might be unusually large or unusually small.
The only way a woman can be fairly certain of the exact day she became pregnant is if she only had one sexual encounter during her fertility window, anywhere from 11-21 days after the first day of her last period, and she knows the exact day that encounter took place.