Prenatal diagnosis and outcome of subamniotic hematomas.

Subamniotic hematomas are classical placental pathological lesions resulting from the rupture of chorionic vessels near the cord insertion. Most subamniotic hematomas are found after birth and result from excessive traction on the umbilical cord at delivery. The development of these lesions has been rarely reported in utero. We report here five cases of subamniotic hematomas diagnosed by ultrasound between 18 and 30 weeks of gestation. The sonographic features were those of a poorly reflective oval-shaped cystic mass overlying the fetal plate of the placenta and covered in a thin membrane. In all the cases, a cystic structure containing a thrombotic mass arising from the amniotic membrane, which was attached to the fetal placental surface, was found after delivery. A high maternal serum alpha-fetoprotein level was found retrospectively in the four cases for which this information was available. Two pregnancies were complicated by slow fetal growth, one by vaginal bleeding and one by polyhydramnios. These findings demonstrated that subamniotic hematoma can be accurately diagnosed in utero by ultrasound and differentiated from other lesions of the placental chorionic plate. The findings also show that, although the lesion is located between the chorion and the amnion, it can be associated with fetomaternal hemorrhage and with fetal growth restriction.