Female Puberty

When girls are between seven and 13 years old, their breasts and nipples start to develop, according to the Palo Alto Medical Foundation. It usually takes between three and five years before their breasts become fully grown, reports the Center for Young Women's Health.

Male Puberty

It is common for boys to develop enlarged breasts temporarily during puberty because of hormone imbalances, according to the MassGeneral Hospital for Children.

Pregnancy

During pregnancy, women's breasts often enlarge from milk production and hormone changes, reports the Microsoft Encarta Encyclopedia.

Weight

When men and women gain weight, their fat tissue produces more estrogen, which may cause them to develop enlarged breasts, according to an article by Jessica Bennett for Newsweek.

Aging Process

Between the ages of 50 and 80, men's bodies often start to make less testosterone, causing a hormone imbalance and enlarged breast tissue, reports the Mayo Clinic.

Drugs

Steroids can cause men's bodies to produce less or no testosterone, resulting in breast enlargement. Male-to-female transsexuals undergoing estrogen and anti-androgen treatments often develop larger breasts, although it take about two years "to reach maximum size," according to TransGenderCare.

Source:

Palo Alto Medical Foundation: Puberty-Changes for Females

Center for Young Women's Health: Breast Health

MassGeneral Hospital for Children: Breast Development

More Information:

Newsweek: Off Their Chests

Mayo Clinic: Gynecomastia

TransGenderCare: Transgender Medical Feminizing Program