The Promised Reformer (Musleh Maud) saga starts from the same time as Muhammadi Begum, in fact in the same advertisement. Even before Mirza Ghulam Ahmad claims to be the re-incarnation of Jesus - he is attributing Messianic qualities to this son. Most likely it was an enticement to the family of Muhammadi Begum that any son born to her would inherit his spiritual estate: