An app which algorithmically monitors female fertility has become the first app to be officially approved for use as contraception – and could now be prescribed on the NHS alongside condoms and the pill. Developed by two Swedish physicists, Natural Cycles uses body temperature to measure fertility. After ovulation, increased levels of progesterone make womens' bodies up to 0.45°C warmer. Users input their daily temperature into the app, which compares their readings against those in its dataset. If the app determines it’s safe to have unprotected sex it will show a green day in its calendar. Days when it’s not safe to have unprotected sex are shown as red – on these days, users need to find an alternative source of contraception, such as condoms, the pill and intrauterine devices.