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You need to visit a doctor in person as there can be many reasons for the rash in pregnancy. If it's associated with fever...we have to check for any viral infection. If there's no fever then get LFTs done and also check for any allergic reaction...or it can be PUPPP's( which is the most common rash in pregnancy). Meanwhile you can use any soothing lotion like histocalamine or lactocalamine for relief of symptom. But if rash persists or increases it's important to get yourself evaluated properly.