Most of the time, you won't know the exact day you got pregnant. Your doctor will count the start of your pregnancy from the first day of your last menstrual period. That's about 2 weeks ahead of when conception happens.
Here's a primer on conception:
Each month inside your ovaries, a group of eggs starts to grow in small, fluid-filled sacs called follicles. Eventually, one of the eggs erupts from the follicle (ovulation). It usually happens about 2 weeks before your next period.
After the egg leaves the follicle, the follicle develops into something called the corpus luteum. The corpus luteum releases a hormone that helps thicken the lining of your uterus, getting it ready for the egg.
After the egg is released, it moves into the fallopian tube. It stays there for about 24 hours, waiting for a single sperm to fertilize it. All this happens, on average, about 2 weeks after your last period.
If no sperm is around to fertilize the egg, it moves through the uterus and disintegrates. Your hormone levels go back to normal. Your body sheds the thick lining of the uterus, and your period starts.
If one sperm does make its way into the fallopian tube and burrows into the egg, it fertilizes the egg. The egg changes so that no other sperm can get in. At the instant of fertilization, your baby's genes and sex are set. If the sperm has a Y chromosome, your baby will be a boy. If it has an X chromosome, the baby will be a girl.
The egg stays in the fallopian tube for about 3 to 4 days. But within 24 hours of being fertilized, it starts dividing fast into many cells. It keeps dividing as it moves slowly through the fallopian tube to the uterus. Its next job is to attach to the lining of uterus. This is called implantation.
Some women notice spotting (or slight bleeding) for 1 or 2 days around the time of implantation. The lining of the uterus gets thicker and the cervix is sealed by a plug of mucus. It will stay in place until the baby is ready to be born.
Within 3 weeks, the cells begin to grow as clumps, and the baby's first nerve cells have already formed.
A pregnancy hormone known as hCG is in your blood from the time of conception. This is the hormone detected in a pregnancy test. Usually it takes 3 to 4 weeks from the first day of your last period for the levels of hCG to be high enough to be found by the tests.