PLACENTAL BASAL PLATE MYOMETRIAL FIBERS - CLINICAL CORRELATIONS OF ABNORMALLY DEEP TROPHOBLAST INVASION

Objective: To assess the incidence of placental basal plate myometrial fibers in preterm and term gestations and correlate this finding with clinical observations and placental histopathology. Methods: Placenta s from 457 singleton births before 32 weeks' gestation and 108 uncompl icated singleton births after 37 weeks' gestation were examined histop athologically. Pregnancies complicated by maternal chronic hypertensio n, diabetes mellitus, coagulopathy, placenta previa, stillbirth, multi ple fetuses, and fetal congenital anomalies were excluded from both gr oups. In the preterm group, 158 patients had preterm labor with intact membranes, 192 had preterm premature rupture of membranes (PROM), 31 had placental abruption without hypertension, and 76 had preeclampsia. Histopathology detected the presence of placental basal plate myometr ial fibers, placental vascular lesions, and villous damage related to vascular insufficiency. Results: Forty-four of 457 (9.6%) of preterm p lacentas had basal plate myometrial fibers, compared with one of 108 ( 0.9%) term controls (P <.001). Uteroplacental vessels with abnormal ph ysiologic changes were more frequent and placental weights were lower in cases-with basal plate myometrial fibers (P <.003 and P <.03, respe ctively). No other uteroplacental vascular lesions were related to bas al plate myometrial fibers. The frequency of placental basal plate myo metrial fibers was nine of 76 (12%) in cases complicated by preeclamps ia, 21 of 192 (11%) cases of PROM, nine of 158 (5.7%) cases of preterm labor, and four of 31 (13%) cases of placental abruption without hype rtension; these frequencies were not significantly different, and ther e was no significant relationship to gravidity, parity, mode of delive ry, or birth weight. Conclusion: Placental basal plate myometrial fibe rs occur in ten times as many preterm births as term births. This find ing is associated with both abnormal uteroplacental physiologic change s and decreased placental weight, and may explain the increased incide nce of abnormalities of the third stage of labor associated with prete rm delivery.