Vasa praevia is bleeding from a vessel in the fetal membranes, for example in the case of a velamentous insertion, of the umbilical cord, or where there is a succinate lobe connected to the main placenta by vessels in the membranes.
The blood loss is from the fetus and this causes acute fetal distress which can only be treated by caesarian section and transfusion of the neonate.
A small amount of the vaginal blood may be mixed with a dilute solution of sodium hydroxide to test for the presence of fetal cells. Alternatively, a blood smear stained with Wright's stains has a fetal element if nucleated cells are seen.
It is this complication that adds the risk to otherwise harmless procedures such as artificial rupture of membranes (ARM).
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