What size muslin should I go with?

My daughter is 20 months and I'm planning her birthday outfit (I need plenty of time to sew since I don't have that much time with a full time job besides parenting). Her chest measurement is 19 but her waist measurement is 21 (as of a few weeks ago). What size should I maker for her? In other pdfs I've been making the 18-24 size but looking at the finished measurement chart it seems like she fits more in a 12-18. Because her birthday is 4 months away I want to account for growth but not end up with something baggy. Thanks!