Birds and Bees—How Sex Works
How an egg becomes fertilized and development begins
During puberty, your body gets ready to possibly become pregnant. Although you are not ready to become a parent, it is important to know how this works.
The process of a woman becoming pregnant and giving birth begins with fertilization. Fertilization is when the egg and sperm join.
It used to be that fertilization happened only through sexual intercourse between a man and a woman. But modern medicine has made it possible for fertilization to happen in other ways, too.
First we'll describe how fertilization happens through sexual intercourse. During sexual intercourse, the man's penis is inserted into the woman's vagina. After a while, the man ejaculates (during orgasm). At this time, the fluid (called semen) containing millions of sperm is released from the opening at the tip of the penis. Some of the sperm "swim" through the vagina into the uterus and then into the fallopian tubes. During sexual intercourse, the woman may also experience the feeling of orgasm, but it's not necessary for fertilization to take place.
Other fertilization methods
Fertilization can also happen through "artificial insemination." With artificial insemination, the man has already released the sperm. The sperm is injected into the vagina—usually by a doctor or nurse. The other type of fertilization is called "in vitro." This is actually a medical procedure. Eggs are taken from a woman's body first. In the lab, sperm are added to the eggs. Some fertilized eggs begin to develop into an embryo. One or more embryos are then injected into the woman's uterus. Women who have difficulty getting pregnant may use in vitro fertilization because it increases the chances for pregnancy.
After the egg is fertilized, it moves through the fallopian tube into the uterus. As it "travels" it is also continuing to develop. Once the egg reaches the uterus, it attaches to the uterine lining (called the endometrium). When that happens, the endometrium is not released from the body and a woman does not have her menstrual period. This is why "skipping a period" or "being late" can mean that the woman has become pregnant.
The fertilized egg attaches to the endometrium and develops into an embryo (the name for a newly-fertilized egg). After seven weeks of development, the embryo becomes a fetus.
A normal pregnancy lasts for 38 weeks (about nine months) from fertilization. During this time, as the fetus grows, the uterus grows, too. At birth, the strong muscles of the uterus, as well as those around the vagina and perineum, plus the muscle of the perineum itself, push the baby through the cervix and vagina and out of the woman's body.