It is not permissible for
a barren woman to visit the graves aiming to become pregnant. This is because such a practice is a sort of appealing to the dead for help. There can be no doubt that appealing to the dead for help is a sort of major Shirk (associating others with Allah in His Divinity or worship that takes the Muslim out of Islam) and a false belief. The only one a woman may appeal to in order to become pregnant and for other needs is Allah Alone. This is because Allah (Exalted be He) says,
And your Lord said: “Invoke Me, I will respond to your (invocation).
Allah (Exalted be He) also says,
...so invoke not anyone along with Allâh.
Also, it is not permissible to slaughter cows aiming for a woman to become pregnant, as this is a sacrifice for other than Allah. Such a practice is a sort of major Shirk (associating others with Allah in His Divinity or worship) in case it is used to seek the satisfaction of the Jinn (creatures created from fire) and other human beings. If it is slaughtered to seek Allah's pleasure for the woman to become pregnant, it will be a Bid`ah (innovation in religion). Anyway, it is not permissible for a Muslim to commit such practices. Moreover, it is obligatory for the person who did such practices to repent and not do them again, with the necessity of warning people against such practices.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.