Within several days of implantation the surrounding endometrium is converted to decidua. These newly formed cells play an important role in regulating placental development. By locally surpressing maternal immune responses, the decidua permits the placental trophoblast to grow. Decidual cells also secrete specific factors that may either stimulate or inhibit trophoblast proliferation. Prior to 8 weeks’ gestation the chorionic sac is surrounded by villi. The eventual position and shape of the placenta results from the degeneration of villi. There is considerable variation in the speed with which villi degenerate, resulting in the chorion laeve. The remaining villi (chorion frondosum) become the placenta. By 17 to 18 weeks’ gestation, the differentation between chorion frondosum and chorion laeve is complete. At mid-gestation the placenta occupies approximately 50% of the uterine surface. After the second trimester, the placenta grows more slowly than the uterus, occupying between a quarter to a sixth of the uterine volume at term. A reduced placental volume in the second trimester precedes fetal complications and growth restriction by at least 3 weeks. The incidence of placenta previa at term is approximately 0.5%. The reported maternal mortality with placenta previa is 0.03% with uterine hemorrhage and disseminated intravascular coagulopathy being the immediate causes of death. Women with placenta previa have an increased risk of abruptio placentae (rate ratio: 13.8), fetal malpresentation (rate ratio: 2.8) and post-partum hemorrhage (rate ratio: 1.7). Placenta previa is associated with advanced maternal age, and multiple pregnancies. Predisposing factors to placenta previa include prior cesarean section, uterine curettage, and maternal cigarette smoking.
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.
The most common diagnosis that must be distinguished from placenta previa is a subchorionic hemorrhage that extends over the internal cervical os. Color Doppler is helpful in distinguishing between these two possible diagnoses.
Transvaginal sonography has also been utilized to define low-lying placentas more accurately.
Oppenheimer et al reported that a transvaginal measurement of > 2.0 cm from the placental edge to the internal cervical os predicts a safe vaginal delivery; all 7 cases with a distance < 2 cm from the internal cervical os to the placental edge within 4 weeks of delivery had a cesarean section for vaginal bleeding.