The objective of this study was to examine the effect of maternal antidepressant exposure on first- and second-trimester maternal serum markers for aneuploidy. We conducted a 10-year retrospective cohort study within a large health care organization. Pregnant women diagnosed with depression who underwent serum screening for aneuploidy were identified. Antidepressant exposure was defined by a filled prescription. Levels of pregnancy-associated plasma protein-A, alpha-fetoprotein, estriol, inhibin, and second-trimester human chorionic gonadotropin (hCG) were obtained, expressed as multiples of the mean. We compared levels of serum analytes between women who were and were not exposed to antidepressants. Using recorded Patient Health Questionnaire scores, we assessed depression severity as a confounder. Antidepressant exposure occurred in 52% of 19,186 pregnancies. Mean inhibin levels were significantly higher in the unexposed group (1.124 multiples of the median compared with 1.084 multiples of the median, P=.003) as were mean hCG levels (1.188 multiples of the median compared with 1.165 multiples of the median, P=.007). Mean estriol levels were lower in the unexposed group (1.005 multiples of the median compared with 1.015 multiples of the median, P=.030). There were no statistically significant differences in the mean values of pregnancy-associated plasma protein-A or alpha-fetoprotein. In bivariate analyses, there were no interactions between analyte values and depression severity. There were no significant differences in the proportion of patients with one or more abnormal level of serum analytes between exposed and unexposed groups. Antidepressant exposure affects mean levels of inhibin, hCG, and estriol. As a result of our large number of participants, small differences could be detected. Further research is necessary to determine if such differences are of clinical significance.