"Underdeveloped lungs" refers to lungs that have not fully grown or developed as they should for a person's age. This can lead to difficulties in breathing and other respiratory problems. Full definition
Like the puppy born with underdeveloped lungs in the birth that I witnessed, sometimes nature never intended for a puppy to make it past the first few breaths of air outside the womb. (ilrdb.com)
They had to stay in the NICU for 11 days because they each had one slightly underdeveloped lung, and some eating issues... (birthwithoutfearblog.com)
Infants born early are more likely to be low weight and suffer complicated health problems including underdeveloped lungs. (webmd.com)