There’s a “third form of twinning,” say researchers.

By Jessica Migala
Updated: February 28, 2019

Add this to cool medical stuff you didn’t know: "Semi-identical twins" are incredibly rare—and a remarkable case has just been described in a new report in the New England Journal of Medicine. Doctors say this is just the second instance of semi-identical twins that have been reported.

In med-speak, this phenomenon is called “sesquizygotic multiple pregnancy.” There are twins that are identical, those that are fraternal, and “semi-identical” (sesquizygotic), which is somewhere in between.

Identical twins occur when one sperm fertilizes one egg, and then the fertilized egg splits, Christine Greves, MD, an ob-gyn at Winnie Palmer Hospital for Women & Babies in Orlando, Florida, tells Health. Fraternal twins occur when there are two eggs that are each fertilized by their own sperm.

In the case of semi-identical twins, two sperm were both able to penetrate one egg before it sealed up. “The egg then split, making two babies,” says Dr. Greves. (For an awesome visualization of this, the BBC has a handy chart.)

In the NEJM article, a 28-year-old woman, pregnant for the first time, naturally conceived twins. A series of DNA testing revealed their truly rare start. As the study notes, the twins shared 100% of their DNA from mom but 78% of their DNA from dad. (To put that in perspective, identical twins share 100% of their DNA, and fraternal twins share 50% of their DNA, same as traditional siblings.)

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For such a rare event, there were potential problems ahead. Dr. Greves notes that the report says doctors counseled the parents about the risk of potential genital ambiguity. (This means a baby’s external genitalia don’t appear to be clearly either male or female, according to the Mayo Clinic.) “These twins progressed until 33 weeks when the doctors found a growth discordance—when two fetuses are differing sizes or weights—something that can happen in any twin pregnancy,” says Dr. Greves.

The twins were delivered via C-section. When the babies were born, one was clearly male and one was clearly female—there was no sexual ambiguity. Though they were healthy at birth, the female twin had a couple of pretty serious health hurdles ahead of her that she overcame. Happily, “both twins were developmentally normal,” the authors write.

If you’re trying for baby, you’re no doubt assaulted with crazy, weird, and frightening stories of what can happen during pregnancy. Don’t file this into your things I have to worry about folder. “It’s a good idea to be plugged into a good ob-gyn system that can take care of you no matter what arises, but rest assured this is incredibly rare,” says Dr. Greves.

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