Fast breathing that slowly becomes normal It is thought to be caused by slow or delayed reabsorption of fetal lung fluid. Before birth the lungs are filled with fluid. Some of this fluid is squeezed out as the infant travels through the birth canal, while the rest must be absorbed by the baby during the first minutes to hours outside of the womb. In babies with TTNB, excess lung fluid reduces the lung function. TTNB is more common in babies delivered by cesarean delivery and in those who are slightly preterm.