Bereshit 30:10

ותלד זלפה AND ZILPAH BARE — In the case of all Jacob’s wives there is mention of their having been with child (i. e. Scripture states ותהר) except in the case of Zilpah. This was because she was the youngest of all and quite a child in age so that her being with child was not noticeable (Genesis Rabbah 71:9) Laban had given her to Leah in order to deceive Jacob — that he might not perceive that they were bringing Leah to him in marriage — for thus was the custom: to give the elder handmaid to the elder daughter, and the younger handmaid to the younger daughter.

And Zilpa Le᾽a’s maid bore Ya῾aqov a son.

וַתֵּ֗לֶד זִלְפָּ֛ה שִׁפְחַ֥ת לֵאָ֖ה לְיַעֲקֹ֥ב בֵּֽן׃