Why does the Torah not mention pregnancy by Zilpah, as it does by the other mothers?
Rashi: Because Zilpah was very young
and her pregnancy was not discernable.
Bereishis Rabah (71:9): And she was not so proper for Bi'ah. This is why Leah did not say 'Bo Eleha' (like Rachel did), and the Torah does not say that Yakov had Bi'ah with her.