Various studies in mice have linked exposure to the environmental compound bisphenol A (BPA), which is common in plastics, to altered DNA methylation. Susiarjo et al. studied the effects on mice of continuous maternal exposure to BPA during oocyte maturation through to the first 9.5 days of embryonic development (an important period of epigenetic reprogramming). DNA methylation and gene expression analyses revealed that exposure to environmentally approved levels of BPA resulted in loss of imprinting (LOI) at some genetic loci in placentas. At higher levels of BPA, LOI also occurred in embryos, and histological abnormalities were observed in placentas. These effects were absent when exposure to BPA occurred only after the period of embryonic reprogramming.