There are mainly 3 types of maternity waistbands to accommodate your growing belly.
Bottoms with Over-the-Belly Panel
Bottoms with Mid-Belly Panel
The answer is when your clothes do NOT fit you anymore. Do yourself a favor and go out and buy properly-sized maternity clothing; you deserve to be comfortable and feel & look great during this very special moment in life! Properly sized maternity clothing fits your body perfectly with accommodations made for your growing chest and belly. And please do not go out and buy oversized non-maternity clothing; it will only make you look bigger than you really are.
For Maternity Clothing Sizing - please choose your pre-pregnancy size!