חסידות אויף יהושע 5:9

1. A woman who cannot conceive should look at the circumcision knife after the circumcision (t.n. The commentators do not provide any citations for this, so I suggest a hint from Joshua 5:9, where we find that after Joshua had all of Israel circumcised G-d says that here He has removed the disgrace, and we find by the birth of Joseph that a woman not being able to bear children is considered to have an aspect of disgrace, so by looking at the knife that removed disgrace there...).