Sizes run small so that the undergarment fabric holds your pump securely in place, making it so your pump doesn't bounce around a lot when you walk. If you are a 28" waist in "regular" clothes or jeans, the 28" waist or 30" waist SkirtMyPump slips would both likely work best for you, depending how tight you prefer your slips to fit.
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*Please see the "How to Guide" blog post on the home page for detailed information about how to best utilize your SkirtMyPump slip*
