Compared with intrapartum diagnosis, antepartum diagnosis greatly improves the perinatal survival rate, which ranges from 97 to 100 percent (Oyelese, 2004; Rebarber, 2014; Swank, 2016). Thus, vasa previa is ideally identiied early, although this is not always possible. Clinically, an examiner is occasionally able to palpate or directly see a tubular fetal vessel in the membranes overlying the presenting part. Efective screening for vasa previa begins during scheduled midtrimester sonographic examination. In suspicious cases, transvaginal sonography is added and shows cord vessels inserting into the membranes-rather than directly into the placenta-and vessels running above the cervical internal os (Fig. 6-7). Routine color Doppler interrogation of the placental cord insertion site, particularly in cases of placenta previa or low-lying placenta, may aid its detection. With this, the vessel waveform relects the fetal heart rate. In one systematic review, the median prenatal detection rate was 93 percent (Ruiter, 2015).
Once vasa previa is identiied, subsequent imaging is reasonable because 6 to 17 percent of cases ultimately resolve (Rebarber, 2015; Swank, 2016). Bed rest apparently has no added advantage. Antenatal corticosteroids can be provided as indicated or given prophylactically at 28 to 32 weeks' gestation to cover possible urgent preterm delivery. Antenatal hospitalization may be considered at 30 to 34 weeks to permit surveillance and expedited delivery for labor, bleeding, or rupture of membranes. Data supporting is limited, and admission may best serve women with risk factors that portend early delivery (Society for Maternal-Fetal Medicine, 2015). A few cases
FIGURE 6-7 Vasa previa. Using color Doppler, an umbilical vessel (red circle) is seen overlying the internal os. At the bottom, the Doppler waveform seen with this vasa previa has the typical appearance of an umbilical artery, with a pulse rate of 141 beats per minute.
of antepartum fetoscopic surgery with vessel laser ablation are described (Hosseinzadeh, 2015; Johnston, 2014). However, current practice is early scheduled cesarean delivery. Robinson mend elective cesarean delivery at 34 to 35 weeks' gestation to balance the risks of perinatal exsanguination versus preterm birth morbidity. he Society for Maternal-Fetal Medicine (2015) considers planned cesarean delivery at 34 to 37 weeks' gestation reasonable.
At delivery, the fetus is expeditiously delivered after the hys terotomy incision in case a vessel is lacerated during uterine entry. Delayed cord clamping is not encouraged.
In all pregnancies, otherwise unexplained vaginal bleeding of vasa previa and a lacerated fetal vessel. In many cases, bleed ing is rapidly fatal, and infant salvage is not possible. With less hemorrhage, however, it may be possible to distinguish fetal ver sus maternal bleeding. Various tests may be used, and each relies on the increased resistance of fetal hemoglobin to denaturing by alkaline or acid reagents (Odunsi, 1996; Oyelese, 1999).
• Knots, Strictures, and Loops