Deciding future dress size based on ttc??

So next week I’ll be going bridesmaid dress shopping for my friends wedding that takes place in May 2020. My hubby and I have decided to start ttc in December. How do you plan for the size of dress to order? I could potentially be 4-5 months pregnant even though not super likely but still could happen. Not sure if knowing we will be trying should affect how I order my dress size. Anyone else ever been in a situation similar?