There are several different ways the zygosity of Registry members is determined. These are:

• Self-report: Upon registration twins or their parents are asked what their zygosity is and how they know this information i.e. doctor, DNA test, their parents told them, or they can tell by the way they look.

• The Peas-in-a-Pod (PIP) questionnaire (see below): Adult twins and parents of twin children who complete our Health and Lifestyle Questionnaire answer a set of PIP questions. The responses to these questions are used to determine zygosity and update the twins’ membership information, (link to PIP article below)

• DNA zygosity test (see below): If the twins have a DNA zygosity test we will update the twins’ membership information with the results.

Twin pairs who have conflicting zygosity reports or don’t know their zygosity are classified as unknown.