Fertilization

Fertilization occurs when a sperm and an egg meet. This can happen naturally, following sexual intercourse, or in a laboratory, through assisted reproduction (ART). For fertilization to occur, the sperm must penetrate 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.

A pregnancy test is often taken when a menstrual 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).