How Twins Are Formed (And More Multiples)

A fertilized egg splits into two… and then splits again.

The miracle of multiple births.

Embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

Embryogenesis starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo.

How Twins Are Made? (Triplets, Quads)

How common are twins and multiples?

As of 2010, twins accounted for about 1 in 30 births in the United States – or 3.3 percent. And 1 in 726 births resulted in triplets or higher-order multiples.

Over the past 30 years, the birth rate for twins rose by about 76 percent. The rate of triplets and higher-order multiples quadrupled from 1980 to 1998, but then started to decline.

What caused the rise in multiples? Part of it was due to women tending to wait until they were older to have a baby. As you get older, hormonal changes make it more likely that your body will release more than one egg at a time. And more than one fertilized egg often means more than one baby. ~BabyCenter

Identical twins occur when a single fertilized egg splits into two. Identical twins look almost exactly alike and share the exact same genes. Most identical twins happen by chance.


Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm. Fraternal twins do not share the exact same genes — they are no more alike than they are to their siblings from different pregnancies. Fraternal twins tend to run in some families.


For Statistics on Twins, visit 

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