According to a majority of the doctors at AskTheDoctor.com,
The best time to do a self-breast exam is 10-14 days after the last menstrual period (Day 1 of previous period). It is during this time the hormones are at its lowest levels so real masses (not lumpy lobules) can be detected. When doing a self breast exam, raise your hand above your head then using the other hand, feel for masses on the breasts. Do the same for the other breast. Breast cancer in your age group is very rare but it does happen. However, benign (non-cancerous) breast masses are common in women from all age groups. To be sure, have it checked by a doctor so a proper breast exam can be done and from there, a decision whether or not to do an imaging procedure (mammogram or ultrasound) can be made.
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