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 lining of the female’s uterus. This is when pregnancy begins.

Semen is the white, sticky fluid that comes out during ejaculation, and it contains hundreds of millions of sperm. When a couple has penile-vaginal sex without a condom, semen is deposited into the vagina. The sperm swim through the cervix and uterus into the fallopian tubes. If the partner with a uterus has recently ovulated (released an egg from one of the two ovaries), then the sperm can join with the egg. When the sperm and egg join, it is called fertilization.

An egg is released from an ovary about once a month through a process called ovulation. An egg can be fertilized for 12 to 24 hours. Sperm, on the other hand, can live for five to seven days inside the reproductive system of a partner with a uterus. That means a couple can have intercourse on Saturday, the partner with a uterus 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, a pregnancy has begun. 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 the reproductive system of a partner with a uterus 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 period is the shedding of this lining.

Pregnancy can also happen by alternative fertilization, where a health care provider extracts an egg and fertilizes it with sperm in a lab through a process called in vitro fertilization (IVF).