That's so odd. I see nothing wrong with telling students you're having a baby. But I suppose your husband is right to leave it be so you don't risk creating issues with the principal and your job. Is there a parent-teacher night coming up? You could share with the parents and they can tell their kids.
Your principal sounds like an idiot and also her talking about your hips/shape is rude and unprofessional. Good thing you only have 8 more weeks