Fetal cystic adenomatoid malformation of the lung: apparent improvement in utero.

Fetal cystic adenomatoid malformation is a rare pulmonary abnormality, usually involving only a part of the lung, that is characterized by excessive growth of the terminal respiratory elements. The natural history of this lesion and hence the prognosis after antenatal detection is still unclear. We report two cases of large cystic adenomatoid malformation, diagnosed prenatally, in which the size of the mass diminished visibly during the third trimester. Both fetuses had excellent outcomes after surgery. These findings suggest that when a cystic adenomatoid malformation is diagnosed antenatally, the initial impression concerning the size of the mass and final prognosis may not necessarily predict outcome, because there may be improvement during fetal life.