The baby's lung issues were caused by the prematurity of its development.

prematurity 是由形容词 premature（早熟的、过早的）加上名词后缀 -ity 构成的名词形式。pre- 是前缀表示"在...之前"，mature 意思是"成熟的"。prematurity 指"未成熟状态"，在医学上特指"早产"的情况。