Vasa previa describes the passage of fetal blood vessels overthe internal os below the presenting part of the fetus. It canoccur with a velamentous insertion, in which the fetal blood vessels insert into the membranes between the amnion and chorion instead of into the placenta and are not protected by Wharton jelly (Fig. 21.4), or when there is a succenturiate lobe across the os from the main pla-centa. Vasa previa occurs in 1 in 2500 pregnancies. Rupture of a fetal vessel occurs rarely in pregnancy, but the risk is greatest with vasa previa. Rupture of a vessel can quickly lead to fetal death, as fetal blood volume is so small. Fetal mortality approaches 60% if rupture is not detected before delivery. When performing artificial rupture of membranes, it is important to ensure that no pulsating vessels are present which may represent a vasa previa.
An Apt test can help distinguish fetal blood from maternal blood.
This test mixes the blood specimen with water to achieve hemolysis. The centrifuged supernatant is mixed with sodium hydroxide (NaOH). Fetal blood remains pink and maternal blood turns yellow-brown.