The Artscroll Family Hagadah explains that Echad Mi Yodeya is asking "in what merit were the Jews redeemed from Egypt?" Well, they believed in one God, they would receive two tablets, they had great ancestors, and so on ... "nine are the months before a baby's born" -- they kept having babies and carrying pregnancies to term, even when things looked bleak.
That last point feels somewhat, well, modern. Is there an earlier commentary that says this, or was this an insight of the Artscroll authors?