Prenatal diagnosis of membranous ventricular septal aneurysms and their association with absence of atrioventricular valve 'offsetting'.

Congenital aneurysm of the membranous portion of the ventricular septum in association with absence of atrioventricular valve 'offsetting' was diagnosed in two fetuses at 29 and 34 weeks. In the first case the fetus had a normal karyotype and no other structural heart defects, whereas in the second case there was a partial deletion of the long arm of chromosome 5 and an absent pulmonary valve syndrome. The association of absence of 'offsetting' with aneurysms of the membranous ventricular septum may represent spontaneous closure of ventricular septal defects initially extended to the inlet.