Velamentous insertion

With velamentous insertion, the umbilical cord inserts into the chorion laeve at a point away from the placental edge, and the vessels pass to the placenta across the surface of the membranes between the amnion and the chorion.

One percent of singletons have velamentous insertion; however, this condition occurs in almost 15% of monochorionic twins [26] and is common in triplets.

Velamentous insertion occurs (1) when placental tissue grows laterally, leaving the centrally located umbilical cord in an area that becomes atrophic, or (2) when the cord implants in the trophoblast anterior to the decidua capsularis rather than the trophoblast tissue that is destined to become the placental mass.

Velamentous insertion has been diagnosed by ultrasonography with a sensitivity of 67% and specificity of 100% in the second trimester; [27] first trimester diagnosis is also possible. [28] The condition is associated with a lower maternal serum alpha-fetoprotein (AFP) and higher maternal serum human chorionic gonadotropin (hCG). [29, 30]

Velamentous insertion can cause hemorrhage if the vessels are torn when the membranes are ruptured, most often with a vasa previa (see below). Velamentous insertion of the cord is associated with low birth weight, prematurity, and abnormal fetal heart patterns in labor, [31, 32] as well as hypoxic ischemic encephalopathy (HIE) of the newborn. [33] If detected, fetal growth may be monitored with ultrasonography in the third trimester. Consider an elective cesarean delivery to avoid a vasa previa rupture or fetal distress if the velamentous insertion is in the lower segment. [27]

Vasa previa

Vasa previa occurs when the fetal vessels in the membrane are situated in front of the presenting part of the fetus. This may occur because of a velamentous insertion of the cord or with vessels running between the placenta and a succenturiate lobe. Vasa previa may also exist over the dividing membrane when a second twin has a velamentous insertion of the umbilical cord.

This condition is usually said to occur in 1 in 2000-3000 deliveries, although a recent publication quoted a higher risk of 1 in 365 deliveries based on ultrasonographic diagnosis. [34]

The cause of vasa previa is unknown. Vasa previa may be associated with low-lying placenta, placenta with accessory lobes, and with multiple pregnancies. [35]

Vasa previa occasionally may be felt on palpation and ultrasonographic detection is well described. The Society for Maternal-Fetal Medicine guideline on the diagnosis and management of Vasa previa states that, under current conditions, vasa previa is diagnosed prenatally with an average sensitivity of 93% and a specificity of 99%. [36, 37] Additionally, sinusoidal fetal heart pattern, fetal bradycardia, or fetal heart rate decelerations during labor may all indicate a ruptured vasa previa. [35]

The risk of fetal exsanguination is significant if the vessels are torn when the membranes rupture, with an associated 50-75% fetal mortality rate. As the fetal-placental blood volume approximates 100 ml/kg fetal weight, modest vaginal bleeding may represent a significant portion of fetal blood volume. In addition to bleeding, if compressed during labor, the vessels can cause fetal heart decelerations. Compression of the vessels during labor can also cause the vessels to thrombose.

Cesarean delivery is the preferred mode of delivery for known vasa previa and is mandatory if significant vaginal bleeding occurs. [38] In addition, late preterm delivery may be advisable for vasa previa. The Society for Maternal-Fetal Medicine recommends delivery at 34 to 37 weeks for stable vasa previa. [36, 37] Prenatal diagnosis of vasa previa can markedly improve outcome. In one report, pregnancies diagnosed prenatally had a 97% fetal survival as compared with 48% in those not diagnosed prenatally. [35] Consider endovaginal color flow Doppler ultrasonography to rule out vasa previa for patients with a known succenturiate lobe or velamentous insertion of the cord.