DNA twin testing is the most accurate method of determining the zygosity of your twins. Twins are either “monozygotic” or “dizygotic,” usually referred to as identical or fraternal, respectively. Many parents overlook the importance of knowing the zygosity of their twins. While most testing is performed to satisfy curiosity, there are good medical reasons to know as well. Because monozygotic twins are genetically identical, they may be able to help one another in extreme medical conditions. They have the best chance of blood compatibility and lowest chance of rejection for organ and tissue transplantation, should the need arise. The following is a ten step guide of things you need to know with respect to your twins.

1. Can’t the number of placentas determine zygosity? The number of placentas is difficult for doctors to determine visually, and it is not always an accurate method. Approxi- mately 33% of monozygotic twins have two placentas, and more than 40% of dizygotic twins have one shared placenta. DNA testing is the most accurate method to determine zygosity.

2. Do we have to test blood? No! While blood is an excellent source of DNA for genetic tests, the cheek swab is designed to be non-invasive, harmless, and easy to do at home.

3. What is the most accurate method for determining if my twins are identical? DNA-based twin testing is the most accurate method of determining the zygosity of your twins. Twins are either “monozygotic” or “dizygotic,” usually referred to as indentical or fraternal, respectively.

4. Monozygotic twins begin as one egg fertilized by one sperm. This tiny embryo splits apart into two embryos around the 4th to 8th day of development. These babies’ genetic fingerprints are identical.

5. Dizygotic twins are born from two separate eggs that are growing in the womb at the same time, fertilized by two separate sperm. Genetically speaking, these children share no more genetic informa- tion than siblings born years apart from the same parents. It is possible, actually very common, for these twins to look very similar at birth.

6. To perform a test you can order cheek swab kit from several online resources containing everything needed for collec- tion of DNA from each twin. The easy to use kit includes step-by-step instructions for handling cheek swabs and the DNA collection card. Protective pouches are also provided for safe and secure shipment to the lab.

7. The accrediting body that oversees human identity testing in the United States is the American Association of Blood Banks (AABB). Currently there are only 38 laboratories that have accreditation status to perform relationship testing activities. These 38 labs are required to meet stringent requirements regarding laboratory staff education and experience and continuing education is required annually. All equipment must be maintained and calibrated regularly for accuracy. In addition, periodic inspections and proficiency tests are performed routinely.  If you choose a laboratory that is not accredited for your testing, there is no guarantee these standards are being met.

8. What happens next? After the samples are received, DNA is extracted from each sample. A process called Polymerase Chain Reaction (PCR) is used to amplify specific targets on the DNA chain referred to as “markers.” These specific markers have been chosen from billions of possible locations in a person’s genome for their extremely unique characteristics. They are the same markers used by police and investigating agencies to identify criminals and missing persons.

9. Genetically speaking, the results are interpreted. Where the genetic information of one twin differs from the other at more than two marker locations, the twins are 100% certain to be dizygotic. When the markers match exactly for all tested locations, the twins are monozy- gotic with greater than 99% probability.

10. How do I get the results?  If you choose an accredited lab, DNA Twin Test results are mailed back to you within 7 days from the date that the lab receives your sample.

Most often DNA Twin Testing is used to satisfy curiosity, however there are medical reasons to definitively know zygosity. Because monozygotic twins have the same genetic code, the twins will have very similar tendencies toward developing or avoiding inherited diseases.  It may be important to screen for birth defects which can occur as a result of monozygotic twinning.  Identical twins have a unique opportunity to help each other in extreme medical circumstances. Usually, identical twins have the same blood type so they could share blood in emergencies. In the event of organ transplantation, identical twins would have the best chance for success.