Zygosity Testing

Are you curious about whether you and your twin are identical or fraternal? If you are a male/female twin pair, you are definitely fraternal twins. However, if you are twins of the same sex, you might not be certain.

If you and your twin need zygosity results that are suitable for health-related purposes, please follow up with your healthcare provider to have a test performed by a CLIA-certified* laboratory.

If you want to know your zygosity only for curiosity and not to make medical decisions, there are commercial laboratories that offer direct-to-consumer zygosity testing. “Direct-to-consumer” means that you deal directly with the laboratory, without necessarily involving a doctor or an insurance company. The laboratory will mail you a home collection kit that you can use to collect cells from inside your cheeks. The collection process is easy and non-invasive. After you collect the cells, you will mail them back to the laboratory in the container provided. The laboratory will extract DNA from your cheek cells and use it to create your DNA profile.

A twin zygosity test compares the DNA profiles of a pair of twins at a number of markers (well-studied sequences of DNA) to see whether they match. If many of these markers are the same, then it is highly probable (i.e., greater than a 99% chance) that they are identical twins. If the twins are different at one or more markers, they are considered to be fraternal twins. The more markers studied, the more reliable the zygosity test results are.

Below we provide a partial list of laboratories that offer direct-to-consumer zygosity testing. The Twin Registry does not endorse any of these laboratories; we are simply providing examples of places that offer this service.

Proactive Genetics
Turnaround time for results: 10 working days
CLIA certified?*: No
Cost: $139/pair

DNA Diagnostics Center (DDC)
Turnaround time for results: 3 working days
CLIA certified?*: Yes
Cost: $250/pair

23andMe
Turnaround time for results: 3-4 weeks
CLIA certified?*: Yes
Cost: starting from $69/person


*CLIA certification: sets certain federal standards for quality and competence. In the US, clinical laboratories must be CLIA-certified. Only testing done in a clinical laboratory (CLIA certified) should be used to make a medical diagnosis, or to develop a plan for prevention or treatment. Commercial zygosity testing laboratories do not have to be CLIA-certified,  although many are, because they also offer other testing that can be used for medical decisions. (CLIA = Clinical Laboratory Improvement Amendments)