Anterior vascularity: Placental vessels that travel over the fetal membranes, close to or covering the inner cervical os, without the protection of the placenta or umbilical cord. It is a rare condition with an incidence of between 1 in 5000 and 1 in 2500. The pathophysiology is unclear and presumably there is a partial overlap with placenta praevia. Risk factors include parietal placenta, umbilical cord sail attachment, placenta praevia or hypoplacenta in mid-gestation, and multiple pregnancies. Clinical significance Undiagnosed placenta praevia is associated with more than 60% of perinatal deaths due to fetal-neonatal blood loss from torn placental vessels during spontaneous or manual rupture of membranes and during labor. There is also a theoretical risk of fetal previa crushing or occluding the placental vessels, causing obstruction of fetal blood return. Anterior vascularization is also often associated with preterm delivery and preterm infant complications, and in most cases, is often necessary to avoid stillbirth or stillbirth resulting in medically induced preterm delivery. The most important factor affecting fetal-neonatal outcome is prenatal diagnosis; Oyelese and associates summarized 155 pregnant women with combined previa, of whom 97% of neonates diagnosed prenatally survived, compared to 44% of those diagnosed at delivery. The diagnosis is rarely made because of palpation of the placental vessels or because of fetal tachycardia or fetal heart monitoring showing sinusoidal waves. At present, anterior vessels are frequently detected by antenatal ultrasound. A standard antenatal ultrasound in mid-pregnancy should assess the placental position, the position of the umbilical cord insertion and the number of placental lobes. Cases of low lying placenta, parietal placenta, and umbilical cord sail attachment should be purposefully screened for anterior vessels using transvaginal ultrasound or color Doppler. Typical findings include linear tubular hypoechogenicity at the internal cervical os on grayscale ultrasound, color Doppler showing blood flow signals, and pulsed Doppler showing pulsed fetal vascular signals. Management Because of the lack of high-quality data, the optimal management remains controversial, with the commonly accepted approach being termination of pregnancy by cesarean section before labor and rupture of membranes. Decisional analysis indicates planned cesarean delivery at 34-35 weeks of gestation and does not require assessment of fetal lung maturity. Patients with suspected previa require emergency cesarean delivery if there is vaginal bleeding. The need for hospitalization remains uncertain, and theoretically inpatients with bleeding, labor, and ruptured membranes can receive a timely and life-saving cesarean delivery, and most authors recommend hospitalization at 30-32 weeks of gestation. However the benefit of early hospitalization has not been proven. Repeat ultrasound in late pregnancy is necessary in pregnant women with combined anterior vessels, as close to 15% of anterior vessels disappear in late pregnancy. The optimal frequency of ultrasound in patients with comorbid anterior vessels is uncertain. If an anterior vessel is detected at 20 weeks of gestation, subsequent ultrasound examinations at 28-30 weeks and 32-34 weeks of gestation are necessary to confirm its actual presence. Laser ablation of previa by fetoscopy has been reported; however, the number of cases is too small to estimate the benefits.