Why did St. Monica defer St. Augustine's baptism?

St. Augustine's mother, St. Monica, was very devout, praying for St. Augustine's conversion, even following him to Milan and successfully getting Bishop St. Ambrose to baptize him, Easter 387. But why didn't St. Monica baptize St. Augustine when he was an infant? Was her husband Patricius, a pagan idolator, opposed to this? (How was St. Monica even validly married to a pagan? Mixed marriages, barring a special dispensation, are invalid.) Fr. Alban Butler's biography of St. Monica says: >In his infancy she had ranked him among the catechumens; and once in an illness, all things were prepared for his baptism, but it was deferred. Why did she defer his baptism? It seems an illness would be all the more reason to have him baptized sooner.