Chorionicity is the number of placentae of a pregnancy. This is most accurately determined between weeks 6 and 9 of gestation, sometimes up to week 12. At this time, a thick membrane (two chorions and two amnions) is clearly visible at ultrasound, separating dichorionic twins, which joins the placenta to form a lambda shape. In monochorionic twins, this membrane (two amnions only) is much thinner and joins the placenta to form a “T” shape. Ultrasounds taken later in gestations are less reliable due to the increased crowding of twins in the uterus.

Physical examination of the inter-twin membranes at birth should also be used to determine chorionicity. This will provide confirmation of early ultrasound data and to determination of chorionicity in twins without early ultrasounds information. Dichorionic membranes are thick, opaque and can be pulled apart, whereas monochorionic membranes are thin and semi-transparent.

© A/Prof Mark Umstad and A/Prof Jeff Craig

For a review of the mechanisms and evidence for typical and atypical twinning, please click here

Our studies have found that parents or twins who were incorrect in their assumptions of zygosity were incorrect because of wrong advice by parents or medical professionals, or by their own incorrect assumptions (Cutler et al. 2015).
Some of the myths that exist around twin zygosity are:

• All same-sex twins are identical
• All twins with separate placentas are fraternal
• All IVF-conceived twins are fraternal
• Identical twins must look exactly alike

 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.

Zygosity questionnaires have been developed to determine the zygosity of a pair of twins based on their overall similarity (“as alike as two peas in a pod”) and how often they are confused by other people.

TRA has a Peas-in-a-Pod questionnaire with three questions:

• Peas-in-a-Pod questionnaire for twin children
• Peas-in-a-Pod questionnaire for twin adults

The questionnaire is based on research by S Ooki and can accurately determine zygosity for 93% of twin pairs:

• Zygosity Diagnosis of Twins by Questionnaire
• Zygosity Diagnosis in Young Twins by Questionnaire for Twins’ Mothers and Twins’ Self-reports

TRA have separately validated this questionnaire:

• Why Accurate Knowledge of Zygosity is Important to Twins

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.