O, Rh-negative blood is the universal donor for blood plasma.
Mother is Rh-neg --> she will generate RH-antibodies
1) fetus affected by Anti-Rh if it is Rh-positive
2) even if O-Rh-Positive is given, then still mother's Rh-antibodies will attack transfused blood due to its cells containing Rh+
3) therefore, O-Rh-negative is best