(ABC News) -- Breast feeding, daily moisturizing and hormone replacement therapy can make a woman's breasts appear more beautiful, but smoking, drinking alcohol and having multiple pregnancies can take an aesthetic toll, according to researchers.
A study of identical twins published today in the Aesthetic Surgery Journal, "Determinants of Breast Appearance and Aging in Twins," shows environmental factors play a key role in how a woman's breasts age.
Other factors like higher body mass index (BMI) and larger bra and cup sizes also contribute to accelerated breast aging, according to the study.
An estimated 316,848 women had breast augmentations and 127,054 had breast lifts performed in 2011, according to the American Society for Aesthetic Plastic Surgery.
Now, women can identify lifestyle behaviors that can slow the aging process to avoid surgical intervention, according to the study, which was funded by a grant from the Aesthetic Surgery Education and Research Foundation.
For the last three years, plastic surgeon Hooman T. Soltanian of University Hospitals Case Medical Center and Case Western Reserve University School of Medicine in Cleveland, Ohio, studied 161 pairs of twins.
"It's very rare that both twins have been through the same exact environmental factors throughout life," he said. "The idea was that they have the same [breasts] from a genetic standpoint. If we see a difference, it's more likely to be environmental factors."
Soltanian collected data from consenting women between the ages of 25 and 74 at the annual Twins Days Festival in Twinsberg, Ohio. The average of the study participants was 45.5 years old.
"The twins come from all over the country for a weekend to have fun and celebrate," he said. "We have been using that opportunity to study their breasts. It's not a longitudinal study, but a cross-sectional study."
The study had two parts. First, each set of twins was given a questionnaire on lifestyle habits, such as smoking, drinking, number of pregnancies, use of a bra, stress at work, sports, hormone replacement therapy moisturizing and exposure to the sun. Each twin answered independently.
Then, photos of the twins' breasts were taken "in a secluded area by professionals." Those photos were "subjectively evaluated by independent reviewers."
Soltanian acknowledged there is "no objective measurement" for what makes a breast "beautiful." But researchers looked for skin tone, droopiness, shape and areola size.
With the data, researchers ran a regression model. "One by one, we check for different factors and try to weed out what is making a significant difference and what factors don't," he said.
Moisturizing seemed an "obvious" advantage on a breast's appearance, showing fewer wrinkles, according to Soltanian. "We know from facial analysis that if you take care of the skin, it slows the aging process down."
Those who received hormone replacement therapy after menopause had more attractive breast shape, size, projection, areolar shape and areolar size.
The study seemed to refute myths about the negative effects of nursing a baby, findings that even surprised Soltanian. Even though the size and shape of the areola had suffered, the skin quality was better in women who breast fed.
"All these twins did not breast-feed without being pregnant and pregnancy has a negative effect on breast appearance," he said. "My explanation is that women who breast fed have a different hormonal milieu -- sort of like internal hormone replacement. So even though those were disadvantages, they gained some benefit."
Soltanian, who does reconstructive surgeries for women after breast cancer, said this twin research could be expanded to longitudinal studies that look for environmental influences when one twin has cancer and the other doesn't.
As for the study's importance, he said, "It's obvious to me that breast appearance and breast health as a whole are a major part of female health."
But according to Judy Kuriansky, a sex therapist from Columbia University, there is no discounting the psychological importance of breast appearance to women.
Breasts carry such anthropological importance, she said. "Like the hip size, it's about being fertile."
"Women are obsessed with their breasts at all ages, because men are obsessed with their breasts," she said. "No matter how liberated women feel about their bodies, their breast size is so tied up in their sexuality and self-image."