Ultrasound in placental disorders.

The definition of placenta previa based on ultrasound findings is more practical, and the traditional definition (implantation of the placenta in the lower uterine segment) needs to be revised. The term 'placenta previa' should only be used when the placental edge overlaps or is within 2 cm of the internal cervical orifice in late pregnancy. If the placental edge is located further than 2 cm but within 3.5 cm from the internal cervical orifice, the placenta should be termed 'low-lying'. Unless the placental edge at least reaches the internal orifice at mid-trimester, symptomatic placenta previa in the third trimester will not be encountered. Caesarean section is the recommended mode of delivery for placenta previa at term. Attempt at vaginal delivery is appropriate for low-lying placenta, but the possibility of post-partum haemorrhage should be kept in mind. The incidence of invasive placentation, such as placenta accrete, has progressively risen in the past 3 decades, possibly as a consequence of increasing caesarean section rates. Ultrasound has a sensitivity of 91% and a specificity of 97% for the identification of all forms of invasive placentation. Chorioangiomas are benign non-trophoblastic placental tumours with excessive vascular proliferation within the stroma of chronic villi. They are usually asymptomatic, although occasionally can be associated with adverse fetal outcomes. Chorioangiomas usually appear as well-circumscribed, rounded, hypo-echoic lesions next to the chorionic surface. Iatrogenic delivery or prenatal intervention are two options, if fetal compromise is present. Prenatal detection leads to a dramatic increase in survival compared with those cases unsuspected antenatally.