A non-viable fertilised egg implants itself into the uterus. It’s non-viable as something goes wrong during the fertilisation process at conception, which means the foetus and placenta don’t form properly and a baby doesn’t develop. There becomes an abnormal overgrowth of cells that would normally form the placenta. You may have no symptoms, but some might be bleeding from the vagina, pain or a lot of quick growth around the tummy. Treatment involves removing the abnormal tissue.