Pulmonary hypoplasia is an incomplete development of the lungs, resulting in an abnormally low number or small size of bronchopulmonary segments or alveoli.

[3][4][5][6] Large masses of the neck (such as cervical teratoma) can also cause pulmonary hypoplasia, presumably by interfering with the fetus's ability to fill its lungs.

In the presence of pulmonary hypoplasia, the EXIT procedure to rescue a baby with a neck mass is not likely to succeed.

[16] In a study of 29 fetuses with suspected pulmonary hypoplasia, the group that responded to maternal oxygenation had a more favorable outcome.

[24] Early delivery may be required in order to rescue the fetus from an underlying condition that is causing pulmonary hypoplasia.

However, pulmonary hypoplasia increases the risks associated with preterm birth, because once delivered the baby requires adequate lung capacity to sustain life.

The decision whether to deliver early includes a careful assessment of the extent to which delaying delivery may increase or decrease the pulmonary hypoplasia.

An example is congenital cystic adenomatoid malformation with hydrops; impending heart failure may require a preterm delivery.

[31] In 1920, decades before the advent of prenatal imaging, the presence of pulmonary hypoplasia was taken as evidence that diaphragmatic hernias in babies were congenital, not acquired.