Most multiple pregnancies produce twins; birth of three or more babies is rare. The majority of twins are nonidentical and develop when two eggs are released from an ovary and are fertilized by two separate sperm. These twins may or may not be the same sex. Less commonly, fertilization of a single egg takes place as normal, but the fertilized cell divides to form two identical, same-sex embryos.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.