From what I found (below) I think if both parents are +, then bub will be too.

There are different ways to classify blood. The two major forms of classification include the ABO system and the Rhesus (Rh) type system.Together, they comprise the eight main blood groups.
A person's Rhesus type is determined by a pair of genes, each one inherited from one parent. Blood is either Rh-positive or Rh-negative, depending on whether or not certain molecules are present. A person who is Rh-negative will experience a severe immune system reaction if Rh-positive blood gets into their bloodstream. This can happen during childbirth, if an Rh-negative woman gives birth to an Rh-positive baby. If blood cells from the baby travel across the placenta, the woman's immune system will regard the Rh-positive cells as a threat. Specialised white blood cells will make antibodies designed to kill Rh-positive blood cells. If the woman subsequently conceives another Rh-positive baby, her immune system will flood her child with antibodies. These antibodies then destroy the baby's red blood cells. If left untreated, this can result in severe anaemia or even death.