Sforno on Genesis 30:17/ספורנו על בראשית ל׳:י״ז

1וישמע אלוקים אל לאה, because she had used both natural means, and then, failing to succeed, relied on prayer. She herself is on record as confirming this when she said נתן אלוקים שכרי אשר נתתי שפחתי לאישי, “G’d has given me my reward for having offered my handmaid to my husband. Giving Zilpah to Yaakov was considered by her as a way of relying on natural means of having more children, seeing she had come to the conclusion that she could not have any more from her own womb.