Invasive Procedures in Twins for Prenatal Diagnosis

Risk of birth defects in multiple pregnancies Frequency of monozygotic twins remains constant, while dizygotic twins become more frequent as maternal age increases. In these cases the risk that at least one fetus is aneuploid will be twice the maternal age risk for a singleton while the probability of both fetuses being involved is minimal. Dizygotic twins have a six fold increased risk that one fetus will be chromosomally abnormal. The true incidence of birth defects among twin pregnancies is controversial and it has been suggested that the risk is twice that of singleton. Furthermore prenatal diagnosis of hereditary diseases in dizygotic twins is a complex problem because of the probabilities of transmission of deleterious gene function independently to each fetus. The risk of an autosomal recessive disorder has a 3 in 8 risk of at least one affected fetus and 1 in 8 chance the both will be affected. Monozygotic twin are 100% concordant for genetic abnormalities and 2%-10% are concordant for development of defects. Prenatal diagnosis in multifetal pregnancy may differ in several ways compared to singleton gestations. The obstetric risk following an invasive procedure is likely to be higher. Before the procedure on should discuss with the couple the likelihood of sampling one or both sacs as well as the possibilities of having discordant results. The management and the prenatal diagnosis of a multiple pregnancy are strongly influenced by chorionicity. In a dichorionic pregnancy there are no direct consequences for the co-twin if a fetal demise occurs, but single intrauterine death in a monochorionic pregnancy can have severe repercussions furthermore for the surviving fetus. In fused placentas the chorionicity must be determined by the ultrasonographic appearance of the dividing membrane. The difference in thickness between the thick dichorionic and the thin monochorionic membranes is much more obvious during the first trimester than it is later in pregnancies. Furthermore the presence of an echogenic chorionic tissue projection into the base of the inter-twin membrane (Lambda sign) in the first trimester has been shown to be one of the most specific ultrasound landmarks of dichorionic placentation. In the second trimester the lambda sign is progressively more difficult to visualize and its absence after 20 weeks of gestation should be viewed with caution.