Outcome of first-trimester chorionic villus sampling for genetic investigation in multiple pregnancy

Objectives To evaluate the efficacy of and risk associated with chorionic v illus sampling for genetic investigations in multiple pregnancies, and to e valuate the accuracy of the ultrasonographic detection of chorionicity duri ng the first trimester. Patients and Methods A total of 198 sets of twins and nine sets of triplets from 10 087 consecutive first-trimester pregnancies undergoing chorionic v illus sampling were considered. Gestational age ranged from 7 to 12.6 weeks . Assessment and confirmation of chorionicity was based on a multiplicity o f features. Dichorionicity was established in 169 sets of twins (85.3%) and trichorionicity in all triplet cases, while 29 twins were considered monoc horionic. Chorionic villus sampling was performed transabdominally in all b ut one case, and identification of the placental insertion of the umbilical cord was the main benchmark for sampling. Sampling risks were evaluated by comparing clinical outcome with that of a control population of 63 dichori onic twin pregnancies which underwent no invasive procedure. Results Determination of the presence or absence of the lambda sign led to a correct assignment of chorionicity in all cases, while the presence of a membrane thickness of 2 mm or more reflected a 100% specificity with a 22% false negative rate. Sampling was successfully performed in all cases and i n only four cases (1.0%) were two needle insertions needed. At follow-up no evidence of incorrect sampling was reported. Karyotyping was provided to a ll patients, and in 94.1% of cases both short and long-term culture methods were carried out. No difference in fetal and perinatal losses between the study and control populations was found, bur a higher rate of deliveries be fore 37 weeks and of low birth weight babies was noted amongst controls. Conclusions Chorionicity in twin pregnancy can be determined with certainty between 7 and 12 weeks of gestation; in cases of confluent placentas relia bility is provided by determining the presence or absence of the lambda sig n. This study indicates that first-trimester transabdominal chorionic villu s sampling is a highly efficient, reliable, and relatively safe approach fo r genetic diagnosis in twin pregnancies. Although a precise evaluation of t he relative risks of chorionic villus sampling and mid-trimester amniocente sis in twins must await randomized control studies, the advantages of a fir st-trimester diagnosis to enable early decision-making about selective feta l reduction are obvious.