too soon?

I'm having a gender reveal baby shower and my ultrasound is schedule for the 27th and I'll be 20 weeks. And I plan on having a small gender reveal baby shower on Sunday the 29. Is it too soon? I know I'll only be 5 months but because I'm so tiny im afraid im going to give birth early, and my OB knows for a fact that I will just obviously not when. My LOs birthday is in May and my DH graduation is also in May and I don't want to take from them, and April is a busy month for my family since Easter  as well as June and July! I'm just at the point where I won't have one after all