What does Genesis 30:9 mean?

Previously in this passage, Rachel has responded to her older sister's success in childbearing (Genesis 30:1) by using her servant as a proxy to produce two more sons for Jacob (Genesis 30:3–8). This followed a strategy used by her own grandparents many years before (Genesis 16:1–4). Clearly, according to the language used, Rachel sees the situation as a high-stakes competition for sons and for Jacob's affection.

Any doubt about whether Leah agreed about the competitive nature of their family is answered here. Prior verses indicated that Leah had stopped bearing children, for then-unstated reasons (Genesis 29:35). Seeing that Rachel has "caught up" somewhat, Leah takes the same approach as Rachel and gives Jacob her servant girl Zilpah (Genesis 29:24) as his fourth wife. Soon Zilpah, and by proxy, Leah, will bear to Jacob his seventh son. Leah, just as much as Rachel, participates in something of a "birth race" which results in explosive growth for the family.