I’ve never had kids and I regularly lift weights, but my 36-year-old midsection still has a softer look than it used to. If you’re over 30, you’ve probably noticed a change in your body fat, too. Age-related muscle loss is partly to blame — we lose 3 to 5 percent every decade, beginning in our 30s — but hormones and other factors affect fat storage in females throughout life.

“It looks like all of female development is aimed at accumulating particular kinds of fat,” says Steven Gaulin, a professor of anthropology at the University of California, Santa Barbara. “Female infants are fatter than male infants the day they’re born.”

Fat has a vital role in providing the building blocks for infant brains and triggering puberty — important biological reasons for women to gain weight, says Gaulin. While having kids is one factor that can redistribute the body’s fat stores, even if you don’t have kids, your body will change as you age. Here’s what to expect.

Puberty
During puberty, women produce a fairly large amount of estrogen. This contributes to a decrease in fat around the waist and an increase in the lower body, says Dr. William Lassek, a former assistant surgeon general and co-author of Why Women Need Fat.

An average woman will have at least twice as much body fat as a man of the same weight, says Gaulin, Lassek’s co-author. One cause for women having more body fat than men has to do with reproduction. Pregnant women may need to have fat stores “in the bank” for healthy development of the baby, whose brain is nearly 60 percent fat.

Pregnancy
After a woman gives birth and starts breastfeeding, the body mobilizes fat stored in the butt and thighs for breast milk, says Dr. Lassek. That’s where the baby’s brain-building Omega-3 DHA fat is stored, he says. Mothers tend to start accumulating fat in their abdomen post baby.

These factors might change how the body looks, but the increase in abdominal fat also helps a woman have larger subsequent babies, says Gaulin. Waist size is a predictor of how big an infant will be, and bigger babies, even in modern society, have a better chance at survival, he says.

“First-time moms need to have low [body mass indexes] so they don’t grow their infants too big,” says Gaulin. “But after they have infants successfully, they can afford to grow a bigger infant, giving it a better start in life.”

Age
Women will increase their BMI starting in their teens into adulthood, says Dr. Lassek. While fat storage will increase at a greater rate if you have a child, it’ll most likely happen to childless women as well, he says.

Menopause
When you stop getting your period and enter menopause, lower estrogen levels contribute to a decrease in fat in the lower body and an increase of fat in the waist. While too much belly fat can contribute to chronic diseases, it can also be protective, says Dr. Lassek.

Fat stored in the lower body is anti-inflammatory, but abdominal fat is where our infection-fighting cytokines are stored, he says. These immune proteins are what help us fight disease and heal wounds. So, older women might be trading reproductive fat for survival fat.

“It’s better to have extra fat if you’re dealing with an infection,” says Dr. Lassek.  “The cytokines tend to block your appetite. You’d don’t want to waste energy digesting food. That’s definitely a survival point.”  

Want to combat extra fat storage?
It’s important to take care of yourself with healthy eating and exercise, but don’t fret too much if you store a little more around the middle than you did in your younger years. If you want to protect your heart, increase your metabolism, and prevent muscle loss, make sure to include weight training in your workout regime.

Keri Wiginton is a writer and photographer focusing on issues related to women's health, mental well-being, and feminism. Her work has appeared in the Chicago Tribune, Washington Post, Austin-American Statesman, Tampa Bay Times and Houston Chronicle. Follow her work at www.keriwiginton.com or on Twitter at @keriphoto.