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Little finger-like appendages on the end of the fallopian tube called fimbria lead the egg into the tube, through which it makes its way into the uterus. If, during this trip, the egg encounters some sperm swimming along, then the first sperm to reach the egg and penetrate the hard outer shell, called the zona pellucida, will enter the egg and begin the life-creating process called fertilization.

A fertilized egg continues down the fallopian tube on a journey that takes about three days. During the first 30 hours, the chromosomes of the egg and the sperm merge, and the cells begin to divide. This new entity is called an embryo. When the embryo finishes its journey and enters the uterus (see Figure 1-2), it gets nourishment from uterine secretions, and the cells inside it continue to divide, causing the embryo to grow. Approximately six days after fertilization, the egg “hatches,” emerging from its hard shell and then burrowing its way into the uterine wall, or endometrium.

Illustration by Kathryn Born

FIGURE 1-2: The embryo nests in the wall of the uterus after fertilization. Happy landing!

The embryo releases a hormone called hCG. When the hCG reaches the mother’s bloodstream, it signals that she is pregnant and causes the ovaries to continue producing the hormones estrogen and progesterone, which are necessary to maintain the pregnancy.

If the egg is not fertilized, it passes through the uterus. About two weeks later, the uterus sheds its lining, the endometrium, in a process called menstruation. A new lining then begins to grow, ready to receive a fertilized egg the next month.