What it is: Thanks to your raging hormones, a prego's skin can be extra sensitive. This means things that normally wouldn't cause a reaction, such as wearing new jewelry, taking certain meds, or petting your friend's dog, can leave you with an uncomfortable rash. There are also pregnancy-specific rashes such as PUPP.
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How it affects exercise: Getting hot and sweaty often aggravates the rash, so you might need to choose cooler workouts such as swimming. If the rash is unbearable, see your doctor about getting a prescription to help clear it up.