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These divine womb opening were part of the great baby-making contest in Genesis 29-30.
And when the LORD saw that Leah was hated, he opened her womb. Genesis 29:31
And God remembered Rachel, and God hearkened to her, and opened her womb. Genesis 30:22
There were other barren woman in the Bible, but Leah and Rachel's wombs were the only ones that were explicityly opened by God.