Rarely, the vessels of the cord insert into the membranes at some distance from the margin of the placenta. The vessels may be damaged as the membranes rupture.
Velamentous insertion is seen in about 1% of singleton pregnancies but is more common in multiple pregnancies.
Velamentous insertion places the foetus at risk from exsanguination due to vasa praevia: the vessels usually cross the region of the internal os ahead of the presenting part of the foetus.
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