How are twins conceived?
Identical “monozygotic” twins are produced when a single egg is fertilized by a single sperm, and the egg then splits into two. The babies may share the membranous, or amniotic, sac that surrounds them in the uterus Depending on when the egg splits, they may also share a placenta. Identical twins, therefore, are the same sex and look almost completely alike as they share the same genetic makeup. Non-identical, or “dizygotic”, twins result when two eggs are fertilized by separate sperm at the same time and each therefore has its own individual genetic makeup Each fetus also has its own amniotic sac and placenta.
