While temporary sterility after childbirth lasts on average only a few weeks for a woman who does not breastfeed, it can last for months, or even more than a year, for one who does. But it was unclear how this contraceptive effect of breastfeeding is affected by a mother’s ‘energetic status’ – that includes her nutrition and levels of physical activity. Researchers from the Max Planck Institute for Demographic Research in Germany and the CNRS in France have just shown for the first time on a global scale that the effect of breastfeeding on ovarian function is highly dependent on environmental conditions.