Pregnancy happens when semen enters a girl’s or woman’s vagina. This can happen during unprotected vaginal sex or by alternative fertilization, where a health care provider inserts the semen into the vagina. Semen is a white, sticky fluid that contains hundreds of millions of sperm, which can fertilize an ovum or egg. The sperm swim through the cervix and uterus into the fallopian tubes. If a woman has recently ovulated (released an egg), then the sperm can join with the egg. This is called fertilization. Ovulation usually happens once a month.

An egg can live and be fertilized for about 24 to 36 hours. Sperm can live for up to five days. That means a couple can have intercourse on Saturday, the girl could ovulate on Wednesday, and the sperm could find an egg on Thursday.

Once the sperm and egg connect, they form what’s called a zygote. The zygote begins to grow and by the fifth day it has a new name, a blastocyst. The blastocyst travels along the fallopian tube, dividing and changing as it goes, and eventually lands in the uterus.

The lining of the uterus is full of blood and tissue. The blastocyst attaches to this nutrient-rich lining. This is called implantation. Once that happens, the pregnancy has officially taken root and can begin to grow. The blastocyst becomes a mass of cells that divide and develop into an embryo. The embryo, in turn, becomes a fetus over the next nine months.

If an egg and sperm fail to connect, the egg dies and the lining of the uterus disintegrates. A girl has her period or is menstruating when the blood and tissue that line the uterus pass out of the body through the vagina.