The cord normally inserts centrally into the placental disc, but eccentric, marginal, or velamentous insertions are variants. Of these, eccentric insertions in general pose no identifiable fetal risk. Marginal insertion is a common variant-sometimes referred to as a battledore placenta-in which the cord anchors at the placental margin. In one population-based study, the rate was 6 percent in singleton gestations and 11 percent in twins (Ebbing, 20l3). This common insertion variant rarely causes problems, but it and velamentous insertion occasionally result in the cord being pulled of during delivery of the placenta (Ebbing, 2015; Luo, 20l3). In monochorionic twins, this insertion may be associated with weight discordance (Kent, 2011).
With velamentous insertion, the umbilical vessels characteristically travel within the membranes before reaching the placental margin (Fig. 6-6) The incidence of velamentous insertion approximates 1 percent but is 6 percent with twins (Ebbing, 2013). It is more commonly seen with placenta previa (Papinniemi, 2007; Raisanen, 2012). Antenatal diagnosis is possible sonographically, and with velamentous insertion, cord vessels are seen traveling along the uterine wall before entering the placental disc. Clinically, vessels are vulnerable to compression, which may lead to fetal hypoperfusion and acidemia. Higher associated rates of low Apgar scores, stillbirth, preterm delivery, and small for gestational age have been noted (Ebbing, 2017; Esakof, 2015; Heinonen, 1996; Vahanian, 2015). Accordingly, monitoring of fetal growth is reasonable either clinically or sonographically (Vintzileos, 2015).
Last, with the very uncommon furcate insertion, umbilical vessels lose their protective Wharton jelly shortly before they insert. As a result, they are covered only by an amnion sheath and prone to compression, twisting, and thrombosis.
With this condition, vessels travel within the membranes and overlie the cervical os. here, they can be torn with cervical dilatation or membrane rupture, and laceration can lead to rapid fetal exsanguination. Over the cervix, vessels can also be compressed by a presenting fetal part. Fortunately, vasa previa is uncommon and has an incidence of 2 to 6 per 10,000 pregnancies
FIGURE 6-6 Velamentous cord insertion. A.The umbilical cord inserts into the membranes. From here, the cord vessels branch and are supported only by membrane until they reach the placental disc. B. When viewed sonographically and using color Doppler, the cord vessels appear to lie against the myometrium as they travel to insert marginally into the placental disc, which lies at the top of this image.
(Ruiter, 2016; Sullivan, 2017). Vasa previa is classiied as type 1, in which vessels are part of a velamentous cord insertion, and type 2, in which involved vessels span between portions of a bilobate or a succenturiate placenta (Catanzarite, 2001).
Two other risks are conception with in vitro fertilization and second-trimester placenta previa, with or without later migra tion (Baulies, 2007; Schachter, 2003).