Breast development is a normal part of growing up for girls. It happens in different stages:

• Before birth: Some early breast tissue forms in a baby girl before she is born.
• At puberty: Breasts begin to grow as hormone levels change. This is usually the first sign of puberty in girls.
• During the childbearing years: Breast may continue to change and grow.
• During menstruation: Some girls and women may notice their breasts feel sore or fuller around their period.
• At menopause (later in life): Breasts may become smaller or less firm.

Breast development actually starts before a baby girl is born.

• Around six weeks into pregnancy, a thick line of tissue forms on the baby’s chest. This is called the milk line.
• In the chest area, small buds begin to form. These are called breast buds.
• Inside each breast bud, little tubes start to grow. These will become milk ducts — the tiny tubes that carry milk later in life.
• By the time a baby girl is born, she already has nipples and the early parts of the milk duct system.

Even though breasts aren’t fully developed yet, the basic parts are already in place at birth.

Each month, a woman’s body goes through hormone changes during her menstrual cycle. These hormones can impact the breasts in the following ways:

• Estrogen (in the first half of the cycle) causes the milk ducts in the breasts to grow.
• Progesterone (in the second half of the cycle) helps grow the milk glands.

These hormone changes can cause:

• Breast swelling
• Soreness or tenderness
• A lumpy felling in the breasts (from glands getting ready in case of pregnancy)

If pregnancy does not happen, the breasts return to their normal size and feel after the period starts. Then the cycle begins again.