Descendent migration in vasa previa.

Abstract

‘Placental migration’ involves the translocation of the placental site to the upper uterine segment in the second and third trimesters [1]. Since the uterine isthmus is usually closed during early pregnancy, subsequently opening with advancing gestation, this phenomenon is thought to strongly involve placental migration. Vasa previa is a condition in which the lower velamentous vessels are located over the region of the internal os. We hypothesized that the velamentous vessels in the lower uterine segment (low VCI) might migrate toward the uterine cervix, in opposition to the direction of migration of the placenta (descendent migration), as the amniotic bag expands toward the uterine cervix and opens the uterine isthmus, resulting in vasa previa. It was supposed that a diagnosis of vasa previa should be made after the opening of the uterine isthmus to ensure an accurate antenatal ultrasound diagnosis. To clarify our hypothesis, a prospective cohort study was conducted, in which the condition of the uterine cervix and isthmus and location of low VCIs were carefully observed using transvaginal ultrasound until delivery. The diagnosis of low VCI with vasa previa at 20 weeks of gestation was made in 12 patients (0.4 % of 2,923 total deliveries). Among eight cases of an opened uterine isthmus at 20 weeks of gestation, six cases of vasa previa and one case of low VCI without vasa previa were diagnosed. The findings in these cases were consistent until the time of delivery. The remaining case of placenta previa with low VCI at 20 weeks of gestation progressed to a low-lying placenta with vasa previa at delivery due to atrophy of the placenta around the internal os (Type II vasa previa). On the other hand, among the four patients with a closed uterine isthmus at 20 weeks of gestation, three were diagnosed with low VCI without vasa previa at delivery. However, in the remaining one case with a low VCI, migration of velamentous vessels toward the cervical os with opening of the isthmus resulting in vasa previa was observed during uterine contractions induced by a transvaginal ultrasound examination at 20 weeks of gestation. Figure 1 presents a transvaginal ultrasonographic picture of such descendent migration. Although velamentous vessels were not running on the uterine os before the opening of the uterine isthmus, the velamentous vessels on the amniotic membrane were descending toward the uterine isthmus with the expansion of the amniotic bag after the opening of the uterine isthmus was induced by uterine contractions. In conclusion, in contradiction to the concept of ascendant migration of the placenta, the velamentous vessels in the lower uterine segment occasionally migrated in the direction of the uterine cervix, resulting in vasa previa. However, the location of velamentous vessels was generally consistent until the time of delivery, after opening the uterine isthmus. This phenomenon suggests that the diagnosis of vasa previa should be made following the complete opening of the uterine isthmus. J. Hasegawa (&) H. Takita T. Arakaki A. Sekizawa Department of Obstetrics and Gynecology, Showa University School of Medicine, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142-8666, Japan e-mail: hasejun@oak.dti.ne.jp