Risk of fetal loss after chorionic villus sampling in twin pregnancies derived from propensity score matching analysis

OBJECTIVE To estimate the risk of fetal loss associated to chorionic villus sampling (CVS) in twin pregnancies using propensity score analysis. METHODS This was a multicenter cohort study performed in eight fetal-medicine units in which the leadership were trained at the Harris Birthright Research Centre for fetal medicine in London and the protocols for screening, invasive testing and pregnancy management are similar. The study population of 8581 twin pregnancies undergoing ultrasound examination at 11-13 weeks' gestation, included 445 twin pregnancies that had CVS. The risk of death of at least one fetus was compared between the CVS and non-CVS groups after propensity score matching (1:1 ratio). This procedure creates two comparable groups balancing the maternal and pregnancy characteristics that lead to CVS, in a similar way in which randomization operates in a randomized clinical trial. RESULTS Death of 1 or 2 fetuses at any stage during pregnancy occurred in 11.5% (51/445) of pregnancies in the CVS group and in 6.3% (515/8136) in the non-CVS group (p<0.001). The propensity score algorithm matched 258 cases that had CVS with 258 non-CVS cases; there was at least one fetal loss in 29 (11.2%) in the CVS group and 35 (13.6%) in the matched non-CVS group (OR 0.81, 95% CI 0.48 to 1.35; p=0.415). However, there was a significant interaction between the risk of fetal loss after CVS and the background risk of feal loss; when the background risk was higher, the risk of fetal loss after CVS decreased (OR 0.46, 95% CI 0.23 to 0.90) while in pregnancies with lower background risk of fetal loss the risk of fetal loss after CVS increased (OR 2.45, 95% CI 0.95 to 7.13). The effects were statistically different (p value of the interaction = 0.005). For a pregnancy where the background risk of fetal loss was about 6% (same as in our non-CVS population) there was no change in risk after CVS, but, when the background risk was more than 6% the posterior risk was paradoxically reduced and when the background risk was less than 6% the posterior risk increased exponentially; for example, if the background risk was 2.0%, the relative risk was 2.8 and the posterior risk was 5.6%. CONCLUSIONS After accounting for the risk factors that lead to both CVS and spontaneous fetal loss and confining the analysis to pregnancies at lower prior risk, CVS seems to increase the risk of fetal loss by about 3.5% above the patient's background-risk. This article is protected by copyright. All rights reserved.