Does the RBCs of mother enter foetus?

The answer is: no

Red blood cells (RBCs) of the mother do not enter the fetus. The placenta, which is the organ that connects the mother to the fetus, acts as a barrier between the two circulatory systems. Maternal RBCs are too large to pass through the pores of the placenta, so they remain in the maternal circulation. Fetal RBCs are produced in the fetus's bone marrow and are smaller than maternal RBCs, so they can pass through the placenta and enter the maternal circulation.