Fetal fibronectin is a protein that's normally present in a pregnant woman's uterus. It's thought to be the glue holding the bag of water to the uterus. If your doctor says you need this test, it's likely because you have a condition that puts you at risk for preterm birth, such as you're carrying multiples or you had a previous preterm birth. For the test, your doctor will test a swab of your vaginal secretions to see if there's fetal fibronectin present. If there is, it could be a sign your body is getting ready for labor early and gives your doctor a heads-up that she may need to take measures to help prevent you from delivering prematurely.