Hormone

A hormone called relaxin helps loosen up pregnant women's hips. Without it, the pain of delivery would be unbearable. Its job done, however, relaxing lingers in female bodies for up to a year, when softer ligaments make new mothers more prone to injury , as Jessica Ennis-Hill, an Olympic champion heptathlete, discovered in training after giving birth in 2014. Five years later Dame Jessica started Jennies, a fitness app to help other women perform safe post-natal workouts. It now lets users optimize workouts for the different phases of their menstrual cycles, and has just concluded a successful funding round.