At what point should I ask for a chair at work?

I work at Verizon, I stand for an 11 hour shift everyday and get to sit down for 1hr of that on my break. Other wise I am standing or walking.. My back and feet kill me.. I'm 16 weeks but I don't have a big bump yet so no one takes me seriously (I've even had people ask me if I'm actually pregnant because I miss a month of work due to hospitalization from sever vommiting) I really want a chair but idk when it's appropriate to email hr and see if I can have one