In order to be fertilized, the egg must be penetrated by a single sperm. Although each ejaculate contains millions of sperm, usually only a few hundred reach the fallopian tubes, a journey that takes healthy sperm only a few minutes. If ovulation has recently occurred, the sperm will encounter an egg cell ready for fertilization. Egg cells are covered by a thick, tough, transparent layer called the zona pellucida, which functions as a sophisticated biological security system. The zona pellucida chemically controls the entry of sperm into the egg. Although there may be hundreds of tail-lashing sperm clustered around the egg, only one actually succeeds in penetrating that outer layer. As soon as it does, a chemical reaction automatically shuts out the rest. This reaction prevents the genetic confusion that would occur if the chromosomes of more than one sperm combined with the chromosomes of the egg.

When fertilization takes place, the genetic materials from the sperm and egg combine, and the egg cell begins to divide. While the egg is in the fallopian tube, there is a considerable risk that it may fail to develop. If it survives, it continues to divide, and by the time it reaches the uterus it is a cluster of cells the size of a speck of dust. By the sixth or seventh day after ovulation, the cluster begins to embed itself in the lining of the uterus, which has become thick, soft, and engorged with blood in preparation for nurturing an embryo. The egg cells continue to divide and by the eighteenth day after fertilization the cells that are destined to form the spinal cord can be detected. At this point the cluster of cells can be called a true embryo.