Embryo development in vivo occurs within a tightly regulated microenvironment. However, this is not the case when undergoing IVF. In this instance, embryos are exposed to light sources that are not normally present in vivo, e.g. during microscopic inspection. Previous studies have indicated a potential negative impact of such light exposure on embryo health. For the first time, we conducted a study that carefully accounted for uniform light illumination across the sample, so that the overall optical energy dose applied was consistent between wavelengths and that the wavelengths were narrow band sources in the visible range, thus mimicking light sources commonly used in fluorescence microscopy (470 – 620 nm).
 Preimplantation mouse embryos were exposed daily to blue (470 nm), green (520 nm), yellow (590 nm) or red (620 nm) wavelengths and compared to embryos that were not exposed. We assessed embryo development, DNA damage, and postnatal outcomes. We found exposure to the yellow wavelength significantly impaired embryo development to the blastocyst stage (P < 0.05). While exposure to blue, green and red wavelengths resulted in significantly higher levels of DNA damage when compared to unexposed embryos (P < 0.05). The pregnancy rate was significantly lower when embryos were exposed to the red wavelength (P < 0.05). Interestingly, resultant offspring were significantly heavier when derived from red or yellow light exposed embryos compared to those derived from unexposed embryos (P < 0.01). Towards understanding the effect on offspring weight we assessed intracellular lipid abundance in the embryo. We found lipid abundance to be significantly elevated following exposure to yellow wavelength (1.8-fold, P < 0.0001) but not red. Red and yellow wavelengths are widely considered benign and utilized clinically in time-lapse equipped incubators within IVF clinics. Our results demonstrate the need to re-evaluate these assumptions.