Depending on their size and chemical composition, intravenously injected nanoparticles can cause pregnancy complications in mice, such as smaller uteruses and smaller fetuses (Nat. Nanotechnol., DOI: 10.1038/nnano.2011.41). Scientists already knew that nanoparticles can cross the placental barrier in pregnant mice and have neurotoxic effects on offspring. This latest study, from a team led by Yasuo Yoshioka and Yasuo Tsutsumi of Japan’s Osaka University, gives more detailed information about the size and chemistry of nanoparticles that can lead to problems. The team found that silica and titanium dioxide nanoparticles, with diameters of 70 nm and 35 nm, respectively, led to smaller fetuses. These particles accumulate in the placenta, as well as in the livers and brains of fetuses. C60 fullerenes and silica nanoparticles larger than 300 nm in diameter did not cause any complications. And functionalizing the smaller silica nanoparticles with carboxyl or amine groups abolished any negative effects. Because the gestational systems of mice and humans differ greatly, the researchers caution against drawing any conclusions about the study’s implications on human health. Nevertheless, they suggest that the fetotoxicity of nanomaterials should be investigated more carefully.