It says that Rachel and Leah gave Bilhah and Zilpah, "l'Ishah," and the latter are called "Neshei" (37:2) (wives of Yaakov). Why does it say that Reuven lay with "Bilhah Pilegesh Aviv" (35:22)?
Bechor Shor (to 30:9): Yaakov married them with Kesuvah and Kidushin. 1 When Reuven lay with her, he profaned her, and she is called a concubine.
Bechor Shor: From when they married Yaakov, Rachel and Leah did not make them work. We find that they had their own tents (31:33). They are called "Shtei ha'Amahos" because initially they were slaves.
Why does it say "Shifchasah"? In the preceding verse, she called her "Amasi" (30:3)!
Malbim: Rachel wanted that Bilhah will bear children, and they will be considered Rachel's.