Vasa Previa

Vasa previa is a condition in which fetal blood vessels run through the membranes, unprotected by cord or placental tissue, across or within 2 cm of the internal cervical os. It matters because when the membranes rupture the vessels tear, and the fetus loses blood within minutes, so perinatal mortality is high if undiagnosed and falls sharply with antenatal diagnosis.

• Type I: velamentous cord insertion. Type II: vessels running between lobes of a bilobed or succenturiate placenta.
• Risk factors: assisted conception, low lying placenta or previa in the second trimester, velamentous insertion, multiple pregnancy, bilobed or succenturiate placenta.
• A fetus has a small blood volume, so even a small loss is life-threatening.

Women are usually well until the membranes rupture, when painless fresh bleeding appears with fetal bradycardia or a sinusoidal tracing. Vessels may be felt as pulsatile cords in the membranes on vaginal examination.

• Painless bleeding at the moment of rupture.
• Fetal heart rate falls rapidly or tracing is sinusoidal.
• Differential diagnosis: placenta previa, placental abruption, show in labour, cervical lesion, cord prolapse, uterine rupture.
• Complications: fetal exsanguination, fetal death, severe neonatal anaemia and hypovolaemic shock, hypoxic brain injury, preterm birth.

The approach is screening at the mid pregnancy scan with transvaginal colour Doppler.

• Transvaginal ultrasound with colour Doppler: linear vessels in the membranes over the internal os or within 2 cm of it, not moving with maternal position change; pulsed Doppler confirms fetal arterial flow at 110 to 160 beats per minute.
• Record cord insertion, placental lobes and distance of the vessels from the os; repeat at 28 to 32 weeks.
• When bleeding occurs after rupture: alkali denaturation test on the blood to show fetal haemoglobin where available, without delaying delivery.
• Cardiotocography from 28 to 30 weeks; haemoglobin, group and cross-match.

The principle is antenatal diagnosis, steroids, admission near the facility and planned caesarean section before labour or rupture, with emergency caesarean section if rupture occurs.

• Supportive: avoid vaginal examination and amniotomy; avoid intercourse; admit from 30 to 34 weeks or earlier with bleeding, contractions, short cervix or distance from the facility; cardiotocography at least twice daily in hospital; two cannulae; blood in the bank including O negative blood for the newborn.
• Pharmacological: betamethasone 12 mg IM twice, 24 hours apart, between 28 and 32 weeks; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour until delivery for 24 hours at most before 32 weeks; nifedipine 20 mg oral once then 10 to 20 mg every 6 to 8 hours for up to 48 hours for contractions; cefazolin 2 g IV before caesarean section; oxytocin 10 IU IM at birth; newborn O negative packed cells 10 to 20 mL/kg IV over 30 minutes and sodium chloride 0.9 percent 10 to 20 mL/kg for hypovolaemia.
• Procedures: elective caesarean section at 34 to 36 weeks and 6 days; emergency category 1 caesarean section at once for ruptured membranes with bleeding or fetal heart changes; clamp the cord at once and hand the baby to the neonatal team; neonatal resuscitation with bag and mask and volume replacement.
• Special groups: twin pregnancy with vasa previa has delivery at 32 to 34 weeks; RhD negative women give anti-D immunoglobulin 500 IU IM within 72 hours, increased by 125 IU for each 1 mL of fetal red cells above 4 mL after fetal bleeding.
• Red flags: bleeding at rupture, fetal bradycardia or sinusoidal tracing, contractions, short cervix, pulsatile vessels felt in the membranes.
• Follow-up: scan at 28 to 32 weeks; weekly review from 30 weeks; newborn haemoglobin and clinical review for 24 hours; postnatal review at 6 weeks.