T. Yee Khong

An examination of the maternal vascular response to placentation shows that physiological changes in the placental bed normally extend from the decidua into the inner myometrium. In pre-eclampsia and in a proportion of pregnancies with small-for-gestational age infants (SGA) the physiological changes are restricted to the decidual segments alone. In addition, complete absence of physiological changes throughout the entire length of some spiral arteries is seen in pre-eclampsia and SGA. This new observation is confirmed in a study of basal plates of placentas from abnormal pregnancies. Intraluminal endovascular trophoblast may be seen in the placental bed spiral arteries in the third trimester in pre-eclampsia and SGA, a feature not seen beyond the second trimester in normal pregnancy. These findings point to a defect in the normal interaction between migratory trophoblast and maternal uterine tissues in pre-eclampsia and in SGA.

The incidence of placenta accreta, defined as the abnormal adherence of the placenta to the uterine wall, has been increasing alarmingly in the developed as well as the developing world. There is considerable maternal morbidity and mortality related to the condition. The pathophysiology focuses on the balance between decidualisation on the one hand and trophoblast invasion on the other. Pathological diagnosis relies on the finding of placental villi in direct apposition to myometrium, either in hysterectomy specimens or in placental basal plates.