Amnion rupture sequence with 'exencephaly': MR findings in a surviving infant.

The amnion rupture sequence is a heterogeneous group of congenital deformities in which rupture of the amnion occurs early in gestation. Fragments of the amnion may adhere to, or encircle and constrict, embryonic parts. This may interrupt normal morphogenesis and cause deformity or even mutilation of formed structures; large cranial defects are a common feature. In contrast to other causes of cranial defects, the amnion rupture sequence carries no increased risk of recurrence in subsequent pregnancies.