STUDY QUESTION: Does age of G-1 PLUS v5 embryo culture medium affect IVF outcome? SUMMARY ANSWER: Birthweight of singletons born after IVF showed an inverse association with age of the embryo culture medium, while no association was found between age of culture medium and fertilization rate, embryonic development or ongoing pregnancy. WHAT IS KNOWN ALREADY: It has been reported that IVF culture media can deteriorate during storage, which suggests that the capacity of culture media to support optimal embryo development decreases over time. Some animal studies showed an effect of storage time on embryo development, in contrast to other studies, while the effect of aging culture medium on IVF outcome in humans is unknown. STUDY DESIGN, SIZE, DURATION: We used data on outcome of 1832 IVF/ICSI cycles with fresh embryo transfer, performed in the period 2008-2012 to evaluate the association of fertilization rate, embryonic development, ongoing pregnancy and birthweight of singletons with age of the culture medium (Vitrolife AB G-1 PLUS v5). PARTICIPANTS/MATERIALS, SETTING, METHODS: Age of the culture medium was calculated by subtracting the production date from the date of ovum retrieval. Data analysis included linear regression and logistic regression on continuous and categorical outcomes, respectively. MAIN RESULTS AND THE ROLE OF CHANCE: Age of the culture medium was not associated with fertilization rate (P = 0.543), early cleavage rate (P = 0.155), percentage of embryos containing four or more cells on Day 2 (P = 0.401), percentage of embryos containing eight or more cells on Day 3 (P = 0.175), percentage of embryos with multinucleated blastomeres (P = 0.527), or ongoing pregnancy (P = 0.729). However, birthweight of the newborn was inversely associated with age of the medium(beta = -3.6 g, SE: 1.5 g, P = 0.021), after controlling for possible confounders (day of embryo transfer, number of transferred embryos, child's gender, gestational age at birth, parity, pregnancy complications, maternal smoking, height and weight, and paternal height and weight) and the association was not biased by year of treatment, time since first opening of the bottle or batch variations. This indicates a difference of 234 g in birthweight of newborns for media with an age difference of 65 days. LIMITATIONS, REASONS FOR CAUTION: The results from this study may be specific for the G-1 PLUS v5 culture medium and extrapolation of the results to other media should be done with caution because of the differences in composition and shelf life. WIDER IMPLICATIONS OF THE FINDINGS: Age of G-1 PLUS v5 medium used to culture human embryos affects birthweight of the respective newborn. This could imply that the preimplantation embryo adapts to its in vitro environment with lasting in vivo consequences. Therefore, it is important that companies are transparent about the exact composition of their embryo culture media, which will allow IVF clinics to further investigate the effects of the media or media components on the health of IVF children.