Jansen CHJR, et al. - Researchers examined the predictive accuracy of the distance between the placenta and the internal os of the cervix in the second trimester for third-trimester low-positioned placentas. In addition, they sought to define a cut-off value at which all third-trimester low-positioned placentas are identified. They conducted a prospective cohort study of 958 women who underwent a transvaginal ultrasound between 18 and 24 weeks' gestation and had a distance between the placental edge and the cervix of < 20 millimeters. A re-evaluation was done in all women in the third trimester. Forty-eight of 958 (5.0%) of placentas persisted as a low-positioned placenta in the third trimester. They observed a higher risk of low-positioned placenta in the third trimester among women with a placenta previa in the second trimester vs women with low-lying placentas (37/181 (20%) vs 11/777 (1.4%)). A higher risk was observed for women with posterior placentas vs those with anterior placentas and for women with a history of cesarean section vs those without such history. In order not to miss any abnormally located placentas in the third trimester, the determined cut-off value for the distance between the internal os of the cervix and the placenta in the second trimester is 15.5 millimeters for posteriorly located placentas and this is even smaller, namely, 4.5 millimetres overlying the internal os of the cervix for anteriorly located placentas.