Why does the Torah not mention pregnancy by Zilpah, as it does by the other mothers?
Rashi: Because Zilpah was very young 1 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 Yaakov had Bi'ah with her.
QUESTIONS ON RASHI
Rashi writes: "Zilpah was younger than the others... Lavan had given her to Leah in order to deceive Yaakov, so that he would not realize that he was being given Leah...." How do we know that Lavan did so with that scheme in mind?
Gur Aryeh: Otherwise, why would Lavan have given the younger maidservant to his older daughter? The Torah spells this out to show Lavan's treachery.