Identical differences

Prenatal diagnosis of zygosity by fetal DNA analysis, a contribution to the management of multiple pregnancies. A series of 31 cases

OBJECTIVE

To evaluate the contribution of prenatal diagnosis of zygosity by fetal DNA analysis to the management of multiple pregnancies.

METHODS

Between March 1999 and March 2000, 31 same-sex multiple pregnancies, were referred to our Institute during their second trimester to have amniocentesis done. Fetal DNA variants were studied in addition to karyotype. The main indications for amniocentesis were fetal growth discordance, fetal malformations or selective pregnancy termination. Zygosity results were compared to the first-trimester ultrasound diagnosis.

RESULTS

21 out of the 31 multiple pregnancies were dizygous, and 10 out of the 31 pregnancies were monozygous. First-trimester ultrasound provided information on chorionicity only in 24 out of the 31 cases (77%). Of these 24 reports, 21 proved to be correct (87.5%). Moreover, in 2 cases among 14 pregnancies with growth discordance, DNA analysis ruled out a twin-to-twin transfusion syndrome, while ultrasound could not yield a conclusion. In 2 twin pregnancies with fetal malformations affecting one of the fetuses, DNA analysis made it possible to assess the risk for the other twin. In the 5 cases leading to selective termination, prenatal diagnosis of zygosity had a decisive value for the surviving fetus.

CONCLUSION

Fetal DNA analysis is a simple and highly useful test for prenatal diagnosis of zygosity in the management of complicated multiple pregnancies.

Mechanisms for differences in monozygous twins

Monozygous (MZ) twins are often described as being physically and genetically identical. Clinical determination of zygosity relies on the assumption that any physical differences between a pair of twins imply they are dizygous. Most twin research relies on the assumption that dizygous twins share approximately 50% of the same genes, whereas monozygous twins share 100%. There is, however, increasing evidence to challenge both these assumptions. In this review, we describe a number of intrauterine effects and genetic mechanisms that may result in phenotypic, genotypic, and epigenetic differences between monozygous twins. Newer molecular techniques are resulting in such differences being increasingly commonly recognised. The potential for differences in monozygotic twin pairs is an important consideration for both clinicians and researchers involved in twin work.