I believe St. Therese of Lisieux (1873-1897) observed that even the smallest of deeds done for the love of God has infinite value. She presumed, of course, that those deeds were done as acts of love and not in a superstitious fashion with the thought that the action would ensure a fruitful result (cf. CCC 2111).
If someone were to pray a Hail Mary with great devotion and with the intention that, if it be God’s will, an unborn baby be saved from abortion, yes, God could choose to reward that prayer by giving a mother the grace to choose life for her child. But if the prayer was thought of in a superstitious fashion, the sentiment that a Hail Mary saves an unborn child could be reduced to the same level of "Every time a bell rings an angel gets his wings."