The changes in placental appearances visible on US have been used to grade the placenta. The placenta is normally 3 cm thick and 15—20 cm in diameter.

·         0 - Homogeneous placental echo pattern and a smooth chorionic plate. Grade 0 placenta is normal till 31 weeks of gestation but is not usually seen at term.

 

·         I   - Random echogenic foci become visible within the placental substance and subtle chorionic undulations may occur. Forty percent of placentas at term are grade I.

 

·         II - More focal echogenic areas are present giving a ‘comma-like' appearance. Basal densities have a stippled appearance. The chorionic plate is now becoming irregular (40% of placentas at term).

 

III - The chorionic plate indentations now extend deep into the placenta to the basal layers. Echogenic foci are prominent and may cause distal overshadowing. These echogenic areas may be confluent in basal area (20% of placentas at term). Reaching grade III before 31 weeks may indicate placenta failure.

Placental appearances have been used to assess maturity of the gestation but placental grading is an unreliable indicator of fetal lung maturity.