Introduction

Vasa previa (VP) represents a rare entity of hemorrhage in the second half of pregnancy in which unprotected fetal vessels of Wharton's jelly or placental tissue flow across the amniotic membrane through the internal cervical os and ahead of the fetal presentation¹. The estimated incidence is 0.46-0.6 in 1,000 pregnancies^2,3.

Three variants of vasa previa have been identified: type 1, secondary to a velamentous insertion of the cord; type 2, associated with bilobed or succenturiata placenta; and type 3, where there are one or more boomerang-shaped vessels crossing the membranes along the placental margin⁴.

The risk factors associated with the entity are: placenta previa (OR: 19; 95% CI: 6.1-58), velamentous insertion (OR: 672; 95% CI: 112-4,034), assisted reproductive techniques such as in vitro fertilization and intracytoplasmic sperm injection (OR: 19; 95% CI: 6.6-54), morphological abnormalities of the placenta such as bilobed placenta and succenturiata (OR: 71; 95% CI: 14-349) and multiple gestation (OR: 2.66; 95% CI: 0.80-8.8)³. It should be noted that between 6%-14.3% of cases are detected VP in the absence of risk factors⁵.

The clinical course can be complicated by rupture of the vessels causing fetal hemorrhage or extrinsic compression of the cord causing heart rate decelerations, asphyxia, and fetal death⁶. When VP is not detected prenatally, fetal mortality ranges from 22.5%-100%⁷, but if the diagnosis is made prenatally, the neonatal mortality rate is <1%; one third of patients will require an unscheduled delivery⁸.

Most international guidelines performing routine prenatal ultrasound recommend an ultrasound evaluation of the umbilical vessels and cord insertion⁹. However, universal ultrasound screening for VP is not recommended unless prenatal risk factors are present¹⁰.

We present a case of prenatal diagnosis of vasa previa secondary to velamentous insertion.

Case report

A 38-year-old female patient, G3P1011, 29 weeks 4 days by first trimester ultrasound, with previous vaginal delivery and medical abortion, without pathological or surgical history of interest and who had 5 prenatal controls in a health center, presented with emergency uterine contractions and little bleeding from the vaginal canal. On physical examination she was hemodynamically stable, with a soft, depressible abdomen occupied by a pregnant uterus of normal tone. The vaginal canal showed scanty dark red bleeding. Fetal monitoring was performed and showed two rhythmic uterine contractions in 10 minutes.

Obstetric ultrasound showed a single fetus in breech presentation without morphological alterations, with a heart rate of 142 beats per minute, fetal weight estimated at 1,494 g at the 62nd percentile for gestational age, normal amniotic fluid volume, low insertion posterior body placenta and, in front of the fetal presentation, vessels passing over the cervix were observed (Figure 1). Transvaginal ultrasound revealed cervical length of 35 mm, and color Doppler showed vessels devoid of umbilical cord running longitudinally above the internal cervical os (Figure 2). Pulsed Doppler exhibited an arterial vessel and another fetal venous vessel (Figure 3), which led to the diagnosis of vasa previa with velamentous insertion. She was hospitalized for tocolysis and pulmonary maturation. Five days later she was discharged without uterine contractions or intercurrences. Weekly outpatient follow-up was indicated. The patient attended only two check-ups.

Figure 1 Abdominal ultrasound: Vessels (¶) from the placenta (‡) passing over the cervix (*) are seen in front of the fetal presentation (†). The maternal bladder is visualized (§). 

Figure 2 Transvaginal ultrasound: In front of the fetal presentation (†), color Doppler shows fetal vessels (¶) running longitudinally above the internal cervical os (*). Placenta (‡). 

Figure 3 Pulsed Doppler. A. Fetal arterial vessel. B. fetal venous vessel. 

At 35 weeks and 5 days of gestation, the patient presented in emergency for regular uterine contractions and scant vaginal bleeding. A cesarean section was performed without complications, in which a live male newborn was obtained, 2,512 grams, 37 weeks by Capurro, adequate for gestational age and Apgar 7 at 1 minute and 9 at 5 minutes. Macroscopic examination of the placenta (Figure 4) confirmed the ultrasound findings of vasa previa secondary to velamentous insertion. In-hospital maternal and neonatal evolution was favorable, and the mother and newborn were discharged after 48 hours.

Figure 4 Macroscopic examination of the placenta; in the circle, fetal vessels unprotected from Wharton's jelly are seen running through the amniotic membrane. A. Fetal placental side. B. Maternal placental side. 

Discussion

The umbilical cord normally inserts into the center of the placenta. However, two main types of abnormal insertion have been identified, both of which have clinical significance during labor¹¹. One is marginal insertion when the cord inserts at the periphery of the placental mass, which is supported by very little placental tissue; and the second is velamentous insertion (VI), when the cord attaches only to the amniotic membranes rather than to the placental mass, leaving the umbilical vessels unprotected by Wharton's jelly¹². These vessels are prone to compression and rupture, especially in labor. In the VI, when the vessels pass through the internal cervical os and in front of the fetal presentation or are within 2 cm of the internal cervical os they constitute the type 1 variant of vasa previa (VP1)^5,13.

The VP1 entity represents a rare entity, estimating that 1-50 cases of VI is complicated with VP and is associated with high perinatal mortality rates, particularly when it is not suspected during the prenatal stage^1,11. VI has an incidence of 0.4-11% in singleton pregnancies and increases in twin pregnancies (1.6-40%). In addition, it has adverse pregnancy outcomes, such as small-for-gestational-age newborns, preterm delivery, perinatal death, intrauterine fetal death, and intrapartum complications, including emergency cesarean delivery^12,14.

Most routine prenatal ultrasound protocols include an ultrasound evaluation of the umbilical vessels and cord insertion into the fetal abdomen¹⁵. However, routine ultrasound often fails to detect VI and universal ultrasound screening is not recommended but may be performed in cases where prenatal risk factors are present⁵. Prenatal VI identification is a desirable clinical goal, as these pregnancies have an increased risk of adverse perinatal outcomes and, despite the obvious importance of prenatal VI detection in obstetric practice, only a few studies have focused on the systematic identification of the site of placental cord insertion during prenatal ultrasonography¹⁶.

Prenatal diagnosis of VP is most often made by ultrasound between 18-26 weeks of gestation. On the gray scale, parallel or circular echogenic lines are observed near the internal cervical os; and if associated with color Doppler, pulsed Doppler fetal vessels show an arterial or venous waveform. The accuracy of ultrasound when performed transvaginally combined with color Doppler is 93%, with specificity between 99%-100%^1,17. Prenatal diagnosis is associated with 97%-99% perinatal survival, while the absence of diagnosis increases the risk of hypoxic morbidity 50-fold and the risk of perinatal death 25-fold¹⁸.

Because the vessels are attached to the chorion, rupture of the membranes may result in rupture of the vessels with fetal hemorrhage. Or extrinsic compression of the cord may occur leading to fetal heart rate decelerations which, if prolonged, may lead to fetal asphyxia and subsequent death¹⁰.

For this reason, the goal of management will be to prolong pregnancy in a safe manner, avoiding possible complications related to rupture of membranes or delivery, as well as minimizing the impact of prematurity¹⁹. Although it is difficult to establish the optimal gestational age for delivery, in cases of uncomplicated VP, elective cesarean section is recommended at around 36 weeks gestation, when a low rate of perinatal mortality and morbidity is observed²⁰.