The management and the prenatal diagnosis of a mul­tiple pregnancy is not only related to the number of fe­tuses, but it is strongly influenced by chorionicity. More than 30% are monozygotic twins and nearly 70% are dizy­gotic twins. Monozygotic twins are at high risk of func­tional and structural abnormalities, of which approxi­mately 10-15% would be affected. Abnormalities unique to monozygotic multiple conception include conjoined twinning, fetus in feto, acardia and fetus papyraceous. Since the frequency of monozygotic twinning remains relatively constant with increased age, dizygotic twins become relatively more frequent as maternal age in­creases. In dizygotic twins, each embryo has an independ­ent risk for an aneuploidy, and therefore, the risk that at least one fetus is aneuploid will be twice the maternal age risk for a singleton. The probability of both fetuses being involved is minimal. In monozygotic conceptions both fetuses share the same karyotype, and hence, the risk of an aneuploid fetus is the same as the risk for a singleton at the maternal age. The indications for prenatal diagnosis in multiple pregnancies are the same as in singleton pregnan­cies. These include advanced maternal age, a previous conceptus with a chromosomal abnormality, a parent with a structural chromosome rearrangement, and the presence of gene(s) associated with inborn errors of metabolism. Nuchal translucency thickness measurement is a valid technique in multiple pregnancies. Prenatal diagnosis in multiple pregnancies differs in several ways from that in a singleton gestation, and is strongly influenced by chorionicity. All invasive prenatal diagnostic procedures must be preceded by a detailed ultrasound examination, and each fetus must be carefully examined in order to evaluate the relative position, size and anatomy of each fetus, the location of the placenta(s) and to determine the chorionicity. Amniocentesis for prenatal diagnosis is rou­tinely performed from 15 weeks’ gestation. In an Euro­pean multicenter study, the pregnancy loss rate was esti­mated to be 2.3 and 3.7% before 20 and 28 weeks’ gesta­tion, respectively. Chorionic villus sampling (CVS) has been demonstrated to be safe and efficacious for sampling twin gestations. Transabdominal CVS is technically simi­lar to mid-trimester amniocentesis and is applied more often than the transcervical technique. Since no marker is available to assure sampling of each chorion, continuous ultrasound localization of the tip of the needle or the cath­eter is required. Contamination occurs most frequently when CVS is performed close to the dividing membrane, which contains villi from both chorions. Twin-twin con­tamination can be avoided by using a combination of transcervical and transabdominal techniques. The risk of CVS-associated fetal loss before 28 weeks’ gestation did not seem to differ between twin and singleton pregnancies (4.9 vs. 4.0%, respectively). Since CVS and amniocente­sis have equal risks of pregnancy loss, the question of which procedure is preferable must be addressed. Amnio­centesis is technically easier and more widely available and accepted. Fetal blood sampling in twins does not pres­ent any difference when compared to this procedure in singletons. Umbilical cords insertions must be identified before sampling. Invasive procedures for prenatal diagno­sis in multiple gestations are safe and effective. There are reports suggesting that obstetric safety, efficacy of sam­pling and accuracy of genetic analysis between CVS and amniocentesis is the same at experienced centers.

Key words: Amniocentesis; Chorionicity; Chorionic villus sampling (CVS); Fetal blood sampling.