Vasa praevia is a condition in which there are fetal blood vessels lying in the membranes in front of the presenting part of the fetus. Often this condition is associated with a velamentous insertion of the cord (where the umbilical cord inserts into the membranes rather than into the placenta). The concern is that these vessels may rupture when the membranes rupture, and that if this happens the blood lost would be from the fetus, who is less able to cope with such a loss than the mother. In most cases this condition only becomes apparent during labour and thus poses a great threat to the survival of the baby.