Doctors sometimes err in their diagnosis of twins’ zygosity (twin type). However, it is very important for twins and their families to know if twins are identical or fraternal.  Among the many reasons are better management of twins’ prenatal difficulties, understanding twins’ developmental events and evaluation of their medical life histories. Why are mistakes made? Here is why:

About one-third of identical twins and most fraternal twins have separate fetal membranes (chorions and amnions) and separate placentas. However, close to two-thirds of identical twins share a placenta and chorion, and a very small number also share an amnion. Some identical and fraternal twin pairs show fused placentas, and in such cases fraternal twins might be incorrectly classified as identical. Finally, some presumably, rare cases of fraternal twins with a shared chorion have been described in the medical literature, and are also likely to be incorrectly assigned.

The most accurate measure of twin type relies on twin-twin comparison of about fifteen DNA markers. Complete concordance for all markers indicates identical twins with nearly 100% certainty. That is because there is a very high degree of individuality in the specified DNA regions that are examined, making it very unlikely for two fraternal twins to match across all fifteen markers. Unfortunately, DNA analysis is not routinely offered to new parents of same-sex twins, and the cost of the procedure may be too high for some families to afford.

Other means of distinguishing identical and fraternal twins include specially developed questionnaires. These forms ask questions about similarity in physical traits (e.g., hair color and eye color) and confusion by others (e.g., parents and teachers). Summary scores are able to classify twins correctly in most cases, when compared with the results from DNA studies. When I conducted my doctoral dissertation research some years ago, twin typing was done by extensive blood group analysis. I discovered that my global impression of twin type upon first meeting twins for the first time was more accurate than the impressions of their parents, judgment of their physicians, scores on physical resemblance questionnaires and results from fingerprint analysis. It seems counterintuitive, but people who do not know twins well are better judges of twin type than those who are well acquainted with them. That is because family and friends become aware of subtle difference between twins that help to tell them apart.

I completed a very informative interview with an Arkansas mother, Tiffany Brown, whose supposed fraternal twins “became” identical when they turned two. Based on analysis of a prenatal ultrasound, she had been told by her perinatologist and ultrasound technician that her twins were fraternal, with 80% certainty. When they were born, Tiffany had no trouble telling them apart, although many people outside her family did. She dismissed their comments until her obstetrician and family friend began noticing the twins’ striking behavioral similarities. She urged Tiffany to have her twin girls’ DNA tested if she wanted to know their twin type for certain. It turned out that the twions were really identical. Tiffany and her family now have greater appreciation for the twins’ similar behaviors and attraction to one another. Hopefully, DNA testing will be offered to all parents of same-sex twins in the near future.

Switched at Birth Twins

The July 12, 2015 issue of the New York Times Magazine featured two pairs of switched-at-birth identical twins from Bogotá, Colombia. I researched the twins in March-April with colleague, Yesika Montoya. Plans are to publish our scientific findings in book form.

Separated Sisters

The 1996 gold medal Olympic gymnast, Dominique Moceanu, made news again when she discovered a sister she never knew she had. Born six years later, Jennifer (Jen) Bricker was adopted by another family and raised in a small midwestern town. Jen was born without legs, but became a well-known tumbler and aerialist before meeting her famous older sister. Based on my interviews with Dominique, Jen and another sister, Christina, their story will be featured in an upcoming issue of Psychology Today. It will also be included in the Versus series in mid-September, produced by Rockwell Films. The director of this segment was Joie Jacoby.

You are reading

Twofold

Musical Talent: A Mix of Genes and Environments

Where does musical talent come from?

Personal Correspondence with Dr. Oliver Sacks

A book review by Sacks led to a letter to the NY Times and a fascinating reply.

Do Twins Read Each Other's Minds?

There is no evidence of twin telepathy.