The most accurate method of determining zygosity is with a DNA zygosity test using the DNA probe method. The first step is to collect small amounts of DNA from inside each twin's mouth using a buccal swab (similar to a cotton bud). The sample contains cells, and most of the cells in our body contain a full set of genetic information in the form of DNA. A person's DNA represents a "genetic blueprint" and like a fingerprint, is unique to each individual. At the laboratory, the DNA is extracted from the cell and 12 specific regions of the DNA are amplified by a process known as PCR (polymerase chain reaction) for careful examination. The DNA patterns of the twins are compared to conclusively determine whether the twins are identical or fraternal. Dizygotic twins may share up to a five marker patterns but monozygotic twins will have the same pattern for all twelve.

The laboratory can determine if twins are identical with a reliability of 99.99%

EasyDNA provides DNA zygosity testing and all TRA members get a special discounted rate.