a. The Matriarchs were barren. This was to teach them (1) not to rely upon their own excellency (2) not to ascribe children to the power of idols, because as young girls they happened to have been delivered into the hands of the priests of idols (their parents being idolworshippers), and (3) to pray to God for children.
Why the Matriarchs Were Barren Before They Were Mothers
Gaster, Exempla No. 287