How Pregnancy Occurs

Each month, a mature ovum (commonly referred to as an “egg”) is released by one of a woman’s two ovaries. The ovum is then picked up by the fallopian tube on the same side and advanced through the tube. This slow process continues as the ovum moves in the direction of the uterus. In the meantime, the lining of the uterus thickens to prepare for implantation, should the egg get fertilized.

This egg will remain alive for 72 hours; however, it is only capable of being fertilized for 12-24 hours. Although it is capable of being fertilized for less than a day, this doesn’t mean sex has to take place during those 24 hours. Sperm can actually live in a woman’s body for up to five days, leaving a much larger window for potential fertilization.

When intercourse results in ejaculation, millions of sperm are deposited into the vagina. About 1% of the total sperm survive the swim to the uterus and fallopian tubes. An estimated 300 of these sperm surround the ovum, attempting to penetrate and fertilize it. Once one sperm succeeds, a chemical is triggered that makes the ovum impenetrable to the other sperm.

The sperm and ovum fuse to form a single cell known as a zygote. The zygote starts to divide into more cells, doubling with each division. The zygote, now called a morula, makes its way to the uterus, where it implants in the lining. Upon implantation, it’s called an embryo. It  continues to grow until it’s known as a fetus. The fetus stays in the uterus for the remainder of the pregnancy until birth.

Each month that no fertilization occurs, the ovum and uterus lining are shed with blood, a process known as menstruation.