What Are Identical Twins
Identical twins, also called monozygotic twins, develop from a single fertilized egg that splits into two embryos. Because they come from the same egg and sperm, they share nearly identical DNA.
How Identical Twins Form
A fertilized egg usually implants as one embryo. In identical twinning, the early embryo splits, often within the first two weeks after conception. The timing of the split can affect whether the twins share a placenta or have separate placentas and amniotic sacs.
Genetics, Health, and Development
Although identical twins have almost the same genome, small genetic differences can occur early in development. They are always the same sex and usually look very similar, yet environmental factors can lead to differences in height, weight, and health over time. Research on identical twins helps scientists study the roles of genes and environment in human traits.
For more background on twin formation and genetics, see the overview at National Institute of Child Health and Human Development.
