"Zilpah his maidservant" (Genesis 29:24). She was young, and she was fit for Rachel, yet she was given to Leah through his [Laban's] deceit, in order to show Jacob that this was the maidservant of Rachel. And from where do we learn that she was young? For it is written concerning her "and she bore" (Genesis 30:10), and it is not written "and she conceived" as with Bilhah. But concerning Bilhah it is written "and she conceived" (Genesis 30:4), for she was a grown maiden and the pregnancy was not noticeable in her; therefore, unmarked, she is young. Another interpretation: "and she conceived" (va-tahar) — what is "and she conceived"? It is like a mountain (har); for when a woman is pregnant she appears like a mountain.
Why Zilpah Was Young and Given to Leah Through Laban's Deceit
Midrash Aggadah, Genesis 29:24
את זלפה שפחתו. והיתה קטנה והיתה ראויה לרחל, וניתנה ללאה ברמאותו, כדי להראות ליעקב שזו שפחת רחל, ומנא לן דקטנה היא, דכתיב בה ותלד (בראשית ל י), ולא כתיב ותהר כבלהה, ובבלהה כתיב ותהר (שם שם ד) שהיתה בחורה ואין ההריון ניכר בה הא סתמא קטנה: ד"א ותהר מהו ותהר, כמו הר כשהאשה מעוברת נראית כהר: