Nanoparticles are used in many everyday products because of their innovative properties, but there are concerns that, being smaller than conventional materials, they have the potential to induce unexpected biological effects. Despite the need for the safety of nanoparticles to be assessed, action in this area is still lagging. Several studies, including those by us, have revealed that nanoparticles can reach the placenta and show placental toxicity, but it remains unclear how nanoparticles affect placental function. Here, we attempted to assess the effect of nanoparticles on placental hormone-producing function by using an in vitro forskolin-induced BeWo syncytialization model. BeWo cells were treated with amorphous silica nanoparticles with a diameter of 10 nm (nSP10) either during syncytialization or after being syncytialized by forskolin treatment. RT-PCR analysis showed that nSP10 inhibited forskolin-induced upregulation in the expression level of human chorionic gonadotropin β (hCGβ; gene name, CGB) during syncytialization, but not in syncytialized BeWo cells. Moreover, nSP10 downregulated forskolin-induced elevation in the expression of endogenous retrovirus group FRD member 1 (ERVFRD-1) and syndecan-1 (SDC1), which are involved in cell fusion during syncytialization. These results suggest that nSP10 could suppress trophoblast cell fusion, thus inhibiting the production of hCG in syncytialized BeWo cells.