Use your current size (or pre-pregnancy + 1)

US sizes

Band CUP
A B C D DD DDD
32 XS XS
34 XS XS S M
36 S S M L
38 M M L XL
40 XL XXL
42 XL XXL XXL
44 XXL 3XL
46 XXL XXL 3XL 3XL
48 3XL

UK sizes

Band CUP
B C D DD E F
30 XS XS
32 XS XS S M
34 S S M L
36 M M L XL
38 XL XXL
40 XL XXL XXL
42 XXL 3XL
44 XXL XXL 3XL 3XL
46 3XL

AU sizes

Band CUP
A B C D E F
8 XS XS
10 XS XS S M
12 S S M L
14 M M L XL
16 XL XXL
18 XL XXL XXL
20 XXL 3XL
22 XXL XXL 3XL 3XL
24 3XL