It is well known that pregnancy is associated with fat weight gain. However, the mechanisms whereby fat mass accumulation is controlled during this period are poorly understood. Therefore, we attempted to determine whether human chorionic gonadotropin (HCG), in vitro , influences human adipose tissue development and/or metabolism. For the first time, HCG/LH receptor was characterized in human adipose cells. We also demonstrated that physiological concentrations of HCG, while unaltering both lipolysis and expression of two markers of lipogenesis ( FAS and ADD1 ) in human mature adipocytes, stimulate human preadipocyte growth via the activation of a protein kinase A-independent mitogen-activated protein kinase/c-fos signaling pathway. HCG also moderately increases the preadipocyte differentiation capacityas reflected by enhanced glycerophosphate dehydrogenase activity and expression of key adipogenic transcriptional factors (C/enhancer-binding protein α and peroxisome proliferator-activated receptor γ 2). Finally, HCG significantly stimulates the secretion of the proadipogenic factor, leptin, from human adipose tissue. Taken altogether, these data suggest that the proadipogenic effect of HCG in human preadipocytes contributes to explain why increased fat storage occurs during the first trimester of pregnancy.