You can't rely on your ready-to-wear size when selecting a pattern size. Your best bet is to record your basic body measurements and refer to them when it's time to buy your pattern.
Tutorial: How To Take In (Adjust) Your Pants – the thread
correct pattern size. How to choose the best size of dressmaking
Camas Sew-Along: Choose your size and fit
Selecting Large Prints For Sewing - Threads
Fitting by the Editors of Threads - Sewing & Crafts - Fitting
Authentically Autumn Cotton and Silk Blouse Patterns - Threads
WonderFil Specialty Threads - Comparing Hand Embroidery Thread
Sc Related Stuff Thread Printable Version 3567
TipStitched Tips - Selecting the Right Pattern Size — TipStitched
Tips for Choosing the Correct Pattern Size
Sewing thread sizes and how to choose