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Jan 2010
Today's question came from a great mom-of-twins blogger, ITSTWINSANITY.COM. Make sure you take a look. This mom has TWO (count 'em -- TWO) sets of identicals! Her question was about the causes of identical (MZ) twinning because she had no explanation for her good luck. I went straight to the source for her answer: Dr. Avner Hershlag (who gave me a wonderful interview for my chapter on IVF -- don't miss it: chapter 5 in "One and the Same") is one of New York's top fertility specialists, and he also has the distinction of being Natalie Portman's dad. (He's also an author and screenwriter in his spare time, which makes the rest of us look like sloths.)
Dr. Hershlag's answer: "Identical twinning, i.e. monozygosity, is a random event occurring at a frequency of 3 in 1000 worldwide. The fact that it does not vary between ethnic groups (like fraternal twinning does) speaks against genetic predisposition. Please remember it is the EMBRYO that splits, NOT THE EGG! Below are different scenarios (see diagram)-- splitting at different stages results in the different types of identical twins, shared/separate membranes and placentas. One etiology introduced is reproductive technologies: assisted hatching in IVF leads to a higher twinning rate (I, among others, published it) and blastocysts in IVF have an increased risk of embryo splitting.
So, if a reader tells you there are 2 sets of identical twins in his family, the possibilities are, in order of frequency:
- that one or both sets are not identical (has DNA testing been done?)
- That they are the result of IVF - many couples won't disclose that. Still, having 2 sets of monozygotics would be unusual.
- A true coincidence
Genetics keeps evolving. it may be that sometime in the future a gene (or genes) that increases the chance of an embryo splitting may be discovered. however, lack of ethnic/geographic/familial clusters make such a gene unlikely."
I'm grateful to Dr. Hershlag for taking even more time out of his busy schedule.....And thanks so much to Heather of Itstwinsanity.com!
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