Human babies grow inside their mothers for 40 weeks enclosed in a watery bag that expands as they do. And as the clock ticks during pregnancy, various processes kick in to thin the membrane material that surrounds the baby so that the bag ruptures at the right time to promote labour. But in some cases this happens far too early and can trigger a pre-term birth, and now scientists at Queen Mary University of London have discovered why. And if we know how it works, we might be in a position to discover how to stop it, as Tina Chowdhury explains... Like this podcast? Please help us by supporting the Naked Scientists