You should avoid giving NSAIDs to pregnant women primarily because it results I the prolongation of gestation, or also results in delayed spontaneous labor. It causes the inhibition of contractile PGs within the uterus wall hence resulting in a delay of the labor onset.
Labor is considered as the most critical stage in the period of pregnancy. It is the stage where the child is ready to be delivered along with the placenta and the umbilical cord still attached. Hence, a delay in the onset of labor could result in several complications which can be fatal for both the mother and the baby in severe cases.