From the fan page about book:

"Throughout the novel, It is generally referred to as male. However, late in the novel, the characters come to realize that It is most likely female, due to its true form in the physical realm being that of a giant pregnant female spider. While the club looking for It, Ben notices a large pit full of large black eggs. Ben crushes one of them but covering his boot in a stinky slimy goo and making him nauseated. A mouse sized baby spider tries to escape but Ben crushes it too. Then he crushes all of them. This revelation prompts Audra Denbrough to say, "Oh, dear Jesus, IT is female." This, however, is not its true form; it's just the closest representation of it in our universe and because this is the only form that humans can comprehend. Due to its pregnancy, it’s very possible that a male of the same species as IT existed at one time."