Adverse effects of chorionic villus sampling: a meta-analysis

Meta-analysis is a popular tool for combining evidence from several related studies. The technique is usually used to combine randomized clinical trials, case-control studies or prospective studies where each study has its own exposed and unexposed groups. By including separate 'study effects' (either fixed or random), one can combine information about differences between control and exposed groups, while still allowing for study heterogeneity. In this paper, we extend existing methods to combine studies of disparate designs, where some studies do not include concurrent controls. We apply the methods to a meta-analysis of the association of prenatal testing via chorionic villus sampling with the occurrence of terminal transverse limb defects.