Twin zygosity testing is a type of DNA testing that determines if twins are identical or fraternal. It is a simple and painless procedure that costs around $200.
It is done for different reasons, but primarily to satisfy curiosity of unsure parents. The only other way to know for sure if twins are identical is if they share the same amniotic sac (a.k.a. monoamnionic/monochorionic, or “MoMo” twins). Conversely, the only other way to know for sure if twins are fraternal is if they are of different sexes (boy/girl) or have different blood types. The old school way of determining twin zygosity based on a single placenta or two separate placentas at birth is no longer accurate. This is because identical twins can still have separate placentas, amnions, and chorions due to the timing of the split when the egg was traveling down the fallopian tube and into the uterus. And, although fraternal twins will always have separate placentas, they can fuse together during pregnancy and appear as one at birth. You can already surmise all of the confusing combinations that would leave parents and doctors guessing.
So, do you really need to know your twins’ zygosity? Maybe. Because identical twins share the same DNA, it is a good thing to be sure of for health reasons (i.e. both twins may be susceptible to the same diseases). Due to the constant questions parents get about their twins being either identical or fraternal, zygosity testing can offer a definite answer. And, of course, there are those who pursue a DNA test out of pure curiosity as mentioned earlier. Either way, it’s a test that should be considered.
I have done DNA testing for twin zygosity on my own twins. They had separate amnions, chorions and what appeared to be separate placentas early in the pregnancy (based on ultrasound). But, when they were born, there was only one placenta. They also had the same blood type and were similar in looks. I must say, we were all immensely curious to know if they were identical or fraternal. And, my husband and I were never sure how to answer that inevitable question when asked by strangers. So, we did the test and found them to be identical twins after all. It was fun finding out and nice to have the answer for our own peace of mind, as well as to give to the pediatrician for their medical records.
Testing is very simple to do. A kit can be ordered online from several reputable companies like IdentiGene and Genetic Testing Laboratories Inc. with results in a matter of weeks.