Monozygotic twins

Monozygotic twins develop when one fertilized egg divides into two embryonic cell populations. They share nearly all inherited DNA, though mutations and environmental effects can make them differ.

Embryonic cleavage: Early cell divisions transform a fertilized egg into a multicellular embryo. The divisions that precede separation into two embryonic populations.
Zygote: A zygote is the single cell formed when an egg and sperm fuse. Monozygotic twins begin as descendants of one zygote.
Twin-to-twin transfusion syndrome: Twin-to-twin transfusion syndrome is an imbalance of blood flow between fetuses sharing a placenta. It is a serious complication specific to many monochorionic twin pregnancies.
Dizygotic twins: Dizygotic twins develop from two eggs fertilized by two sperm and are genetically comparable to ordinary siblings. They contrast with twins arising from one fertilized egg.
Chorionicity: Chorionicity describes whether a multiple pregnancy has one or more chorions, the membranes forming the outer fetal sacs. The number of chorions depends largely on when the embryo divides.
Fertilization: Fertilization is the fusion of an egg and sperm to form a zygote. It creates the single starting cell from which the twins develop.
Twin reversed arterial perfusion sequence: Twin reversed arterial perfusion sequence is a rare complication in which one twin's circulation supplies a severely malformed co-twin. Shared placental vessels make this complication possible in monochorionic pregnancies.
Zygosity: Zygosity is the degree to which individuals share genetic origin, especially whether twins come from one or two zygotes. It names the distinction that separates monozygotic and dizygotic twins.
Amnionicity: Amnionicity describes whether fetuses have separate or shared amniotic sacs. Division timing also determines whether monozygotic twins share an amniotic sac.
Meiosis: Meiosis is the cell-division process that produces eggs and sperm with half the usual chromosome number. It generates the parental gametes whose DNA the twins inherit.

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