Study describes new type of "semi-identical" twins by Will Dunham

I have a brother and a sister that are fraternal twins that were born over 18 years ago. But even before this family association, I have been very interested in multiples. Through my mom's child care center, I've know 2 sets of triplets, many sets of twins. And with the increase of fertility enhancement procedures, the percentage of multiples continues to rise. I was very intrigued when this article came up on my screen.

A new study has reported that there is a newly discovered way for twins to form. Initially, the two ways that were identified were where either 2 eggs were fertilized each with a different sperm, and the other way was when one fertilized egg split into two identical forms. These processes create fraternal and identical twins.

This last way that has been discovered found out that some twins develop when one egg is fertilized by two separate sperm. These twins aren't identical but carry more similarities between them than fraternal twins. These types of twins are being referred to as "semi-identical"

The new case came to light because one of the twins had an abnormality in sexual development -- ambiguous genitalia -- and was considered a hermaphrodite with both ovarian and testicular tissue. This child is being raised as a girl. The other twin is a boy.

"Since we don't ordinarily look at the genetics of twins, there probably are more cases out there, but they have not come to light because they haven't been investigated in the way that these twins were," added Souter, a geneticist also trained in obstetrics and gynecology who is the mother of fraternal twins.

The "semi-identical" twins were evaluated at a U.S. hospital initially a few years ago, the researchers said.

According to their doctor the twins were healthy at their last appointment a few months ago.