Since having children in Judaism was considered a great blessing, legitimate wives often gave their maids to their husbands so they could have children with them when those women themselves where childless, normally because of infertility issues as in the cases of Sarah and Hagar; Leah and Zilpah; and Rachel and Bilhah.
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Since it was regarded as the highest blessing to have many children, legitimate wives often gave their maids to their husbands to atone, at least in part, if they were barren, as in the cases of Sarah and Hagar, Rachel and Bilhah.