Sizing for December? 🤔

Kate • Baby #1 due June 2018 ❤️✨

I am due June 14th 2018 and found the shirt I want to order for customization to wear in my announcement photo( 12 weeks starting December)! ❤️ ☃️ However, the t shirt is unisex sizes ( not maternity) and I usually wear a small regularly before pregnancy. Currently at 10 weeks I’m not showing, but wondering what size would be good to order for December? Thanks, ladies, for any input!