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Practice Paper 1: Answers

E – Vasa praevia Vasa praevia is rare, occurring in only 1 in 3000 pregnancies. Normally the umbilical cord comes out of the centre of the placenta but, in vasa praevia, the umbilical cord vessels travel away from the placenta through the membranes and overlie the internal cervical os. The vessels can tear, leading to rapid exsanguination of the fetal circulation. The risk of vessels tearing is greatest when cervical dilation occurs and at rupture of membranes. A severely abnormal cardiotocograph (CTG) is seen with a small amount (<500 mL) of painless vaginal blood loss. Because it is fetal blood that is lost in vasa praevia, fetal mortality is very high (35%–95%), while there is little risk to the mother. Delivery must be expedited, normally with a caesarean section, and the neonate may need transfusion. There have been some studies into using ultrasound scans as a screening modality, but that is not commonly practiced due to limited evidence of benefit and rarity of the condition. Therefore, diagnosis is clinical and only confirmed when the placenta and membranes are examined after caesarean section.