Bledidoc Answers

Which is more common, fraternal or identical twins?

Naturally, twins occur in about 1 in 250 pregnancies.

Birth rate of identical twins is the same all over the world: It is 4/1000 births.

It is a random event, and it is not influenced by race, age, heredity, or history of prior pregnancies.

Birth rate of fraternal twins varies with genetics, maternal age (older women have an increased risk), fertility treatment, races (African women are more at risk than Hispanic or Asian women), number of pregnancies, time of the year (most conceived in July), height (the risk increases with the height of the mother). ^1,2,3

In conclusion, fraternal twins are much more common than identical twins, especially these days with the increased use of infertility treatment.

Dr. Nicolas Kioumji

Pediatric

¹Roberta Spiro, Washington State Twin Registry, TAG Archives

² Shur N 2009. The Genetics of Twinning from Splitting Eggs to Breaking Paradigms. Am J

Med Genet Part C Semin Med Genet 151 C : 105-109

³ Machin 2009. Non-identicla Monozygotic Twins, Intermediate Twin Types, Zygotic

Testing, and the non random nature of Monozygotic Twinning : A review. Am J Med Genet

Part C Semin Med Genet 151 C 110-127

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1Roberta Spiro, Washington State Twin Registry, TAG Archives