placenta previa

diagnosis

from a clinical perspective four types of placental implantation have been defined relative to the internal cervical os: 1.total placenta previa – the entire cervix is covered by placenta. 2.partial placenta previa – the cervical os is only partially covered by placenta. 3.marginal placenta previa – the placental edge extends to the margin of the internal cervical os. 4.low-lying placenta – the lower edge of the placenta is in the lower uterine segment, but does not reach the cervix. these diagnoses are based upon a digital examination of the cervix. they are, therefore, not entirely applicable to the patient who is not in labor and who is examined by sonography. when the cervix is closed it is difficult to distinguish between a total and a partial placenta previa with sonography. the apparent ‘placental migration’ from the region of the cervix with advancing gestational age is due to three factors: 1) the differential growth of the uterine fundus in contrast to the mid and lower uterine segments during the first two trimesters of pregnancy; 2) the rapid thinning and elongation of the lower uterine segment during the third trimester; and 3) the ability of the placenta to change shape and orientation within the uterus by growing in one direction and degenerating in another location. an abnormal implantation, uterine pathology, or a structural defect in the placenta may therefore, singly, or in combination, result in a placenta previa. studies with transabdominal sonography have reported prevalence figures of between 1.9% and 6.2 for second trimester placenta previa. when low-lying placentas are included, rates of 9.1% to 45% have been reported. the rate of false positive and false negative diagnoses of placenta previa in the third trimester is between 2% and 6%, respectively. an overly distended maternal bladder, uterine contractions, or a placental edge that hangs over, but does not impinge upon, the cervical os are some of the technical factors that may result in a false positive diagnosis of placenta previa with transabdominal sonography. transvaginal sonography overcomes many of the inherent difficulties associated with the transabdominal assessment of the lower edge of the placenta. in the first trimester hill et al utilizing transvaginal sonography found a 6.2% prevalence of placenta previa. the likelihood that a placenta previa will persist until term increases if the placenta covers the internal cervical os by > 1.6 cm. farine et al have reported a 100% sensitivity and negative predictive value for the sonographic diagnosis of placenta previa with transvaginal sonography. the only false positive diagnosis of placenta previa in their study occurred when the transvaginal examination was performed prior to 35 weeks’ gestation. leerentveld and co-workers reported a 1% false positive rate and a 2% false negative rate for the detection of placenta previa with transvaginal sonography. in order to exclude placental migration, they recommended transvaginal sonography every 2 weeks in cases of third trimester placenta previa. the incidence of placenta previa diagnosed by transvaginal sonography at 15 to 20 weeks’ gestation is 1.1%; 14% persist until delivery. between 15 and 24 weeks’ gestation, the sonographic detection of a placenta extending over the internal os by > 1.0 cm predicts a placenta previa at term with 100% of sensitivity and 85% specificity. when the placenta is low-lying or just over the region of the internal cervical os, optimal visualization of the placenta edge and its relationship to the internal cervical os is difficult with transabdominal sonography. in these instances an accurate assessment of the placental location with transvaginal sonography will reduce the number of subsequent ultrasound examinations necessary to exclude a diagnosis of placenta previa.