A: Maternity clothing is typically designed for a 25 to 40 pound weight gain, so generally you should buy your pre-pregnancy size. With that said, keep in mind that if you have a quick weight gain in the beginning of your pregnancy, then your size has probably changed. We recommend you read each description carefully. (P) - Petite - pre-pregnancy size 0-2, (5' 4" and under). (S) - Small - pre-pregnancy size 4-6. (M) - Medium - pre-pregnancy size 7-8. (L) - Large - pre-pregnancy size 9-10. (XL) - Extra Large - pre-pregnancy size 11-12. (2X) - Sizes 14-18. (3X) - Sizes 20-22.