Most women are able to become pregnant from puberty, when their menstrual cycles begin, until menopause, when their cycles stop. A pregnancy starts with fertilization, when a woman's egg joins with a man's sperm. Fertilization usually takes place in a fallopian tube that links an ovary to the uterus. If the fertilized egg successfully travels down the fallopian tube and implants in the uterus, an embryo starts growing.

Ovulation, fertilization, implantation

All the eggs for a woman's lifetime are stored in her ovaries. Women do not keep producing eggs. This is different from men, who continuously make more sperm.

About once a month, an egg is released from one of a woman's two ovaries. This is called ovulation. The egg then enters the nearby fallopian tube that leads to the uterus.

If a woman and a man have unprotected sexual intercourse, sperm that is ejaculated from the man's penis may reach the egg in the fallopian tube. If one of the sperm cells penetrates the egg, the egg is fertilized and begins developing.

The egg takes several days to travel down the fallopian tube into the uterus. After it is in the uterus, a fertilized egg usually attaches to (implants in) the lining of the uterus (endometrium). But not all fertilized eggs successfully implant. If the egg is not fertilized or does not implant, the woman's body sheds the egg and the endometrium. This shedding causes the bleeding in a woman's menstrual period .

When a fertilized egg does implant, a hormone called human chorionic gonadotropin (hCG) begins to be produced in the uterus. This is the hormone that a pregnancy test measures. It prevents the uterine lining from being shed, so the woman does not have a period. Other signs such as breast changes and nausea occur in a woman's body, also meaning that pregnancy has begun.