Fetal fibronectin (fFN) is a fibronectin protein produced by fetal cells. It is found at the interface of the chorion and the decidua (between the fetal sac and the uterine lining). It can be thought of as an adhesive or biological glue that binds the fetal sac to the uterine lining.etal fibronectin leaks into the vagina if a preterm delivery is likely to occur and can be measured in a screening test. When the fFN test is positive, it is an inconclusive result. A positive result can indicate that a woman will go into preterm labor soon, but she may not go into labor for weeks.