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Identical or Fraternal? Determining Zygosity in Multiples

You Can't Always Tell by Looking

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Updated July 14, 2014

Newborn twins
Stephen Swain Photography/Taxi/Getty Images

"Are they identical or fraternal?" This is probably the most common question asked of parents of multiples. Many people do not understand what they are really asking, assuming that it is just a matter of twins having similar physical appearance. But twin type is actually determined long before the twins are born. The terms identical and fraternal are common words that refer to zygosity -- the characteristics of the cell union that happened at conception.

Identical -- monozygotic (one zygote) -- twins form when a single fertilized egg splits into two genetically identical parts. The twins share the same DNA set, thus they may share many similar attributes. However, since physical appearance is influenced by environmental factors and not just genetics, identical twins can actually look very different. Identical twins are always same-sex sets.

Fraternal -- or dizygotic (two zygotes) -- twins develop when two separate eggs are fertilized and implant in the uterus. The genetic connection is no more or less the same as siblings born at separate times. They may look alike, or they may not.

Scientists have theorized a third, hybrid type called polar body twinning, that occurs when an unfertilized egg splits into two parts and each part is fertilized by a different sperm. The twins would then share one-half of their gene set (from their mother). Because it is the father's DNA that determines the sex, the twins can be either same-sex or male/female.

So if appearance isn't a reliable guide in determining twin type, how can parents confirm their twins' zygosity? In some cases, it will be evident during pregnancy. Some identical twins form in a single sac, sharing a placenta and amniotic membranes. However, the number of placentas isn't always a clear indicator either. Two placentas of fraternal twins can fuse together and appear to be one. And identical twins may develop with completely separate placentas and sacs.

The only way to know for sure is through genetic testing. A sample of DNA is taken from each child (usually through a painless swab on the inside of the cheek) and compared for similarities. Companies like Proactive Genetics promise a 99% accuracy rate for a cost of about $160.

So why does it matter? Some parents are motivated by medical concerns, while others are participating in scientific research. But most simply want to satisfy their curiosity or confirm their suspicions.

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