Chapter 194 Abnormalities of Placental Implantation
INTRODUCTION
Description: Failure of the normal process of decidua formation results in a placental implantation in which the villi adhere directly to (accreta, 78%), invade into (increta, 17%), or go through (percreta, 5%) the myometrium. One portion (partial) or all (total) of the placenta may be involved.
Prevalence: Difficult to assess; estimates vary from 1 in 1667 to 1 in 70,000 pregnancies (average 1 in 7000).
ETIOLOGY AND PATHOGENESIS
Causes: Abnormal decidua formation at the time of placental implantation. (Imperfect development of the fibrinoid [Nitabuch’s] layer.) Abnormal site of placental implantation (previa, 64% of placenta accreta, cornual or lower uterine segment, or uterine scars such as site of previous cesarean delivery).
Risk Factors: Placenta previa (without previous uterine surgery 5%, with previous surgery 15% to 70%), previous cesarean delivery, multigravidity (1 of 500,000 for parity <3, 1 of 2500 for parity >6), older pregnant women, previous uterine curettage, previous uterine sepsis, previous manual removal of the placenta, leiomyomata, uterine malformation, prior abortion, endometrial ablation.
DIAGNOSTIC APPROACH
Workup and Evaluation
Imaging: No imaging indicated. (Ultrasonography has been used to make the diagnosis before labor in unusual cases. Low-lying placentas noted in studies performed at less than 30 weeks may “migrate,” leaving the cervix free at term [up to 90% of cases].)
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