Fetal echocardiography is a helpful tool in diagnosing and following t
he natural history of fetuses with congenital heart disease. This repo
rt describes two fetuses with complex congenital heart disease that de
veloped progressive pulmonary stenosis in utero. On fetal echocardiogr
am, one of the fetuses had tetralogy of Fallot and the other had a sin
gle ventricle. Both fetuses, when followed serially by fetal echocardi
ography, showed pulmonary stenosis that progressed in severity. These
findings confirm that certain cardiac defects progress in severity in
utero.