Vasa previa

Vasa previa is when placental vessels/fetal vessels, enmeshed only in a thin membrane--as in "velamentous" insertion of the cord (the vessels of the cord separate in the membranes away from the actual placental edge)--and overlie the cervix, being at risk to exposure from outside (exams) or from inside (labor or the descending baby).

Typically, the vessels come together over thick placenta, into the cord near the middle of the placenta. If the cord is placed eccentrically, away from the center, so are the converging vessels that end up making up the cord.

And if the cord comes together from these vessels away from the placenta altogether, this means that the vessels will travel some distance in only membranes, away from the safety of the thickness of the placenta.