Did you ever have anyone give you a hard time about your choice to not find out the sex?

My husband is really excited about the surprise of not knowing, and I was on the fence, so I decided to go with it. My parents have known that we would do this since before I was pregnant. I am now 20 weeks and I have heard my mom complain about it every single time I talk to her, and I talk to her at least once a week, usually more. She mentions it in a "joking" way, like she's teasing me, but at this point it's really starting to get old and it doesn't feel like teasing any more, just like a giant guilt trip. I have already told her once to not give my DH a hard time about it because she was talking about calling him and bugging him until he changed his mind. I guess since she feels like she is complaining to me, and not him that it's okay.

I'm at the point where I feel like I need to say something again, but she can be very sensitive and I need help coming up with a nice way to ask her to stop it that hopefully won't make her defensive. Any suggestions or experience with this?