Fetal bronchoscopy at 29 weeks’ gestation. This allowed for diagnosis and management of a suspected obstructive fetal lung mass. Fetal lung masses, or fetal lung lesions, are defects that develop in the lungs before a baby is born. Some masses may be related to another congenital defect, or a genetic condition, but in most cases the mass is isolated. It is unknown why a baby's lung tissue doesn't develop normally.