Pregnancy begins when an egg is fertilized by a sperm. This event takes place in the outer third of one of the fallopian tubes, when a single sperm meets and fuses with an egg following sexual intercourse. Within 2 days of fertilization, the egg starts its journey towards the uterus, propelled by the muscular action of the fallopian tube. As the egg travels, its cells gradually divide to form a cluster of cells called a morula. After 5–7 days, the morula arrives in the uterus and then embeds securely in the uterine lining. From this moment onwards, the pregnancy is properly established, and the morula is known as an embryo.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.