The Torah tells us (Bereishis 11:30) that Sara was barren. Why was this so? Was she not a righteous woman? Why was it necessary for G-d to later allow her to conceive only in a miraculous way?

G-d wanted to establish Himself as the progenitor of the Jewish people and not Terach, an idolater. G-d ensured that Avraham and Sara could not have children. Otherwise, people would say that their ability to do so came through the natural order and hence Terach would be seen as a forefather of the Jewish people. By allowing Sara to have a child only through a miracle, G-d prevented people from making such an error.