Context Prenatal treatment with dexamethasone to prevent virilization in pregnancies at risk for classical congenital adrenal hyperplasia (CAH) remains controversial. Objective To conduct a systematic review and meta-analyses of studies that evaluated the effects of dexamethasone administration during pregnancies at risk for classical CAH because of 21-hydroxylase deficiency (CYP21A2). Data Sources We searched MEDLINE, EMBASE, and Cochrane CENTRAL from inception through August 2009. Review of reference lists and contact with CAH experts further identified candidate studies. Study Selection Reviewers working independently and in duplicate determined trial eligibility. Eligible studies reported the effects on either foetal or maternal outcomes of dexamethasone administered during pregnancy compared to a control group that did not receive any treatment. Data Extraction Reviewers working independently and in duplicate determined the methodological quality of studies and collected data on patient characteristics, interventions, and outcomes. Data Synthesis We identified only four eligible observational studies (325 pregnancies treated with dexamethasone). The methodological quality of the included studies was overall low. Meta-analysis demonstrates a reduction in foetus virilization measured by Prader score in female foetuses treated with dexamethasone initiated early during pregnancy (weighted mean difference, -2·33, 95% CI, -3·38, -1·27). No deleterious effects of dexamethasone on stillbirths, spontaneous abortions, foetal malformations, neuropsychological or developmental outcomes were found although these data are quite sparse. There was increased oedema and striae in the mothers treated with dexamethasone. There were no data on long-term follow-up of physical and metabolic outcomes in children exposed to dexamethasone. Conclusions The observational nature of the available evidence and the overall small sample size of the whole body of the literature significantly weaken inferences about the benefits and harms of dexamethasone in this setting. Dexamethasone seems to be associated with reduction in foetus virilization without significant maternal or foetal adverse effects. However, this review underscores the current uncertainty and further investigation is clearly needed. The decision about initiating treatment should be based on patients' values and preferences and requires fully informed and consenting parents. © 2010 Blackwell Publishing Ltd.