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How does pregnancy happen?

Pregnancy happens when an ovum, often called an egg (the female sex cell), has been fertilized by sperm (the male sex cell) and the zygote (the organism that results from the joining of an egg and a sperm) implants itself into the female’s uterine lining.

Semen is the white, sticky fluid that contains hundreds of millions of sperm that comes out when a guy ejaculates. When a guy and a girl have unprotected vaginal sex, semen is deposited into the vagina. The sperm swim through the cervix and uterus into the fallopian tubes. If the girl has recently ovulated (released an egg from one of her two ovaries), then the sperm can join with the egg. When the sperm and egg join, it is called fertilization.

Pregnancy can also happen by alternative fertilization, where a health care provider inserts semen into the vagina through a process called in vitro fertilization (IVF).

Girls usually release an egg about once a month through a process called ovulation. An egg can be fertilized for 24 to 36 hours. Sperm, on the other hand, can live for five to seven days inside the female reproductive system. That means a couple can have intercourse on Saturday, the female could ovulate on Wednesday, and the sperm could fertilize the egg on Thursday.

Once the sperm fertilizes the egg, a zygote is formed. This is a cluster of cells that continues to divide as it travels along the fallopian tube. After five days, it is referred to as a blastocyst. When it reaches the uterus, there is a lining of blood and other tissue in the uterus that has been developing in case an egg is fertilized. The blastocyst attaches to this nutrient-rich lining and literally dumps its genetic material into the lining. This process is called implantation. Once that happens, she is medically pregnant. After implantation, the mass of cells is referred to as an embryo, and later, after a couple of weeks, it is referred to as a fetus.

If sperm are deposited into a girl’s reproductive system but no egg is present, or if an egg is present but sperm don’t fertilize it (sometimes this happens), then the egg dies and the lining of the uterus sheds. A girl getting a period is the shedding of this lining.

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