Fertilization occurs when a sperm and an egg meet. For this to happen, the sperm must cross the outer layer of the egg. The egg and the sperm then fuse to form a single cell.

The fertilized egg starts to develop and slowly descends toward the uterus to form an embryo. It will implant itself in the lining of the uterus, which is called the endometrium. Implantation takes place about seven days after ovulation.

Most women take a pregnancy test when they realize their period is late. If the test is positive, it means that fertilization has occurred.

In about one out of every six pregnancies, the embryo will not develop or the baby’s heart will stop beating relatively early on. The uterus will then stop growing and expel its contents, ending the pregnancy in miscarriage (see Miscarriage).

Egg: Reproductive cell produced by the ovary. When an egg and a sperm fuse, an embryo may form.

Embryo: Name given during the first full 10 weeks of pregnancy to the human being developing in the mother’s abdomen.

Fertilization: Fusion of a sperm and egg.

Miscarriage: A spontaneous abortion, which can have a variety of causes (e.g., a deformity or disease).

Sperm: Reproductive cell produced in the testicles. When a sperm and egg fuse, an embryo may form.