Multiple gestation, or multiple pregnancy, occurs when two or more fetuses are conceived at the same time in the same woman. It can be differentiated from the term "multiple births" because it refers to the conception of the fetuses, and may or may not result in the live births of multiple babies. In some cases, the woman’s body may naturally reduce the number of fetuses present, or a woman may decide to reduce the number due to the health risks associated with multiple gestation.
With multiple gestation, the fetuses are either monozygotic or dizygotic. Monozygotic means that, during conception, the sperm fertilizes one egg, which will later split into two or more developing embryos. These types of siblings are genetically identical and almost always the same sex.