Fat?

There's one class of students I never told I was pregnant, because I only see them about once a week. The conversation never came up, so I figured they would figure it out once I started showing. So, here's part of a conversation with one of those students today:

Student: "Mrs. Bunag, are you pregnant?"
Me: "Yes, I just didn't say anything, because it never came up."
Student: "Oh OK. We were wondering, but didn't want to ask in case you were just getting fat!"

P.S. - In case you were counting, I'm 18 weeks along today.