Final answer:
An Rh-negative woman carrying an Rh-positive fetus is most at risk of Rh incompatibility, potentially leading to hemolytic disease of the newborn in her second or subsequent pregnancies due to sensitization in her first pregnancy.
Step-by-step explanation:
The pregnant woman most likely to have a problem with Rh incompatibility with the fetus is actually one who is Rh-negative and is carrying an Rh-positive baby.
Rh factor incompatibility can result in hemolytic disease of the newborn (HDN), where the Rh-negative mother can become sensitized to Rh-positive red blood cells (RBCs). During the first pregnancy with an Rh-positive fetus, there's generally no issue as the mother's primary antibody response takes time to develop, and the initial exposure to the fetus' Rh-positive RBCs often occurs too late in the pregnancy to affect the first baby.
However, if the mother becomes pregnant again with another Rh-positive child, her body has already been sensitized and will produce a quicker, more robust secondary antibody response. The IgG antibodies created can cross the placenta and attack the fetal red blood cells, leading to HDN. This condition can be severe, leading to anemia, brain damage, and sometimes death. Due to modern medical intervention with Rho(D) immune globulin injections, Rh incompatibility problems can often be prevented.