Sforno on Genesis 30:3/ספורנו על בראשית ל׳:ג׳

1בוא אליה, this was Rachel’s answer to her husband’s accusation. She had not meant that it was within Yaakov’s power to make her pregnant, but she wanted to nurse a child by the next best method, that of becoming foster mother to her handmaid’s child.

2ואבנה גם אנכי ממנה, so that I will have a share in posterity just as has my sister through her children.

3ממנה, a recipe found in Megillah 13 for barren women to be jealous of other women who have no problem conceiving in order for their own organs to respond in a manner that helps them conceive.