Pregnancy and Conception

Most of the time, you won't know the exact day when you got pregnant. Your doctor will count the start of your pregnancy from the first day of your last menstrual period. That's about two weeks ahead of when conception actually occurs.

Here's a primer on conception:

Ovulation

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 two weeks before your next period.

Hormones Rise

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.

The Egg Travels to the Fallopian Tube

After the egg is released, it moves into the fallopian tube. It stay there for about 24 hours, waiting for a single sperm to fertilize it. All this happens, on average, about two weeks after your last period.

If the Egg Isn't Fertilized

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.

Fertilization

If one sperm does make its way into the Fallopian tube and burrow 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.

Implantation: Moving to the Uterus

The egg stays in the Fallopian tube for about three to four days, but within 24 hours of being fertilized it starts dividing very 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 one or two 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 the first week, a hormone called human chorionic gonadotropin (hCG) can be found in your blood. It's made by the cells that eventually become the placenta. 

A blood or urine test at your doctor's office will usually detect hCG. It may take three or four more weeks for it to show up as a positive result on a home pregnancy test.

Within three weeks, the cells begin to grow as clumps, and the baby's first nerve cells have already formed.

Pregnancy Hormones

Human Chorionic Gonadotrophin (hCG) is a pregnancy hormone present in your blood from the time of conception; it is produced by the cells that form the placenta. This is the hormone detected in a pregnancy test, but usually, it takes three to four weeks from the first day of your last period for the levels of hCG to be high enough to be detected by pregnancy tests.

The development stages of pregnancy are called trimesters, or three-month periods, because of the distinct changes that occur in each stage.