What is the ruling on purchasing a belt and circumambulating the Kaaba with it, intending that it would help the wearer conceive?

A daughter asking her mother to send a belt after circumambulating the Kaaba with it for blessings is a prohibited form of seeking blessings, as seeking blessings is only legislated for that which is explicitly mentioned in the Sharia. Anything not mentioned in the Sharia, even if it comes from the Kaaba itself, is an innovation (bid'ah). Instead, it is more appropriate to supplicate to Allah alone for good and blessings, such as the blessing of a child. For "To Allah belongs the dominion of the heavens and the earth; He creates what He wills. He gives to whom He wills female [children], and He gives to whom He wills males. Or He combines them [as] males and females, and He renders whom He wills barren. Indeed, He is Knowing and Competent."