There are generally two types of twins: fraternal and identical. Identical twins are sometimes called paternal or maternal twins, but these are non-scientific terms and simply mean that the twins strongly take after either their mother or their father. Although all twins are born from the same womb, fraternal and identical twins form differently.

Fraternal or non-identical twins develop when two separate egg cells get fertilized by two separate sperm cells. Fraternal twins have different physical characteristics and features because they do not share similar chromosomes. Fraternal twinning, the most typical type, accounts for about 40 percent of all twin pregnancies. Fraternal twins can be different or the same gender. Scientists also believe that fraternal twinning is a genetic trait.

Identical twins, like a regular pregnancy, start off as a single egg cell that gets fertilized by a single sperm; however, as the zygote (the fertilized egg) develops, it divides itself in half and forms two embryos that develop into babies. Unlike fraternal twinning, scientists still do not know the exact reason why the zygote splits to form identical twins. Identical twins generally have the same sex, eye and hair color, as well as blood type. Their physical features look typically the same, with most twins having an exact mirror face of the other.

There are some cases where the fathers of twins are different individuals. Although the idea may seem unusual, there are several instances that this occurrence was proven through DNA testing. One such instance occurred in May 2009 in Texas, where a woman gave birth to twin boys having different fathers.