If you're worried about a lump in your breast, talk to your doctor. They stop growing in early adulthood when breasts are fully developed, but start again during pregnancy to make way for milk-producing glands, then transform once more when a mother stops breastfeeding. Learn more on our Support Center. Yet the higher risks of earlier developers ending up with breast cancer or Young teen girl see breast other diseases are a significant concern, not to mention other psychological and physical problems that can crop up. Create a sweet "tweet" for bird friends.