What is the cause of babies not growing well in the womb?

May be you are talking about erythroblastosis fetalis?

this hemolytic disease happens to an unborn infant when the mother and baby have different blood types. The mother produces substances called antibodies that attack the developing baby's red blood cells.

The most common form of erythroblastosis fetalis is ABO incompatibility, which can vary in severity.

The less common form is called Rh incompatibility, which can cause very severe anemia in the baby.