Also called fraternal or dizygotic twins, non-identical twins are generally twins that don’t look alike but are born during the same pregnancy. The association is courtesy two individual eggs or ova getting fertilized by two different sperms. The eggs make two zygotes; therefore, the name dizygotic. In some rare cases, the eggs could fertilize at distinct times with a couple or more acts of sexual intercourse, either inside a single menstrual cycle or later on during pregnancy. Fraternal twins have different placentas and amniotic sacs for growth sustenance. Though rare, there are fraternal twins with identical appearances.  

Non-identical twins. Image credit: Flickr
Non-identical twins. Image credit: Flickr

Of the total twin births, 2/3rd is fraternal; and almost 2 percent child deliveries represent fraternal twins. These twins are like any other sibling pair – the only difference is fraternal twins are born in one delivery or on the same day. Fraternal twins’ DNAs are half similar to each other, also similar to any random brother-sister pair. The twins could be of the same or opposite sex. However, opposite sex twins are likely to be fraternal since they don’t possess identical DNAs.

Causal Factors

Dizygotic twin pregnancies are likely if the mother is/has had:

  • African, or of African descent.
  • Between 30 to 40 years of age.
  • Above average weight and height.
  • Multiple prior pregnancies.
  • Assisted reproductive technology, especially medicines that increase the quantity of eggs released.
  • Heavy amount of sex during fertile times.
  • Diet rich in sweet potatoes and yams.

A fraternal twin is likely if it runs in the family. If a woman’s grandmothers, mother, sisters or aunts have had fraternal twins, she is quite likely to have fraternal twins too. The likelihood increases if the mother herself was a fraternal twin.