It’s a classic lament: Pounds seem to creep on with every passing year. And, unfortunately, the phenomenon is supported by the facts. Starting at around age 40, the average American woman gains about a pound a year, says Lewis Kuller, M.D., an obesity researcher at the University of Pittsburgh. That means by age 60, you may be carting around an extra 20 pounds and courting such problems as heart disease and diabetes.

At first glance, the cause is simple arithmetic: Eat more than you expend and eventually your pants will be too tight. But other factors, from your hormone levels to the size of your dinner plate, can complicate the equation. Chances are the weight gain is caused by physiological and lifestyle changes that you’ve barely noticed.

Knowing more about the four most common changes and understanding how they affect your weight will help you stay slimmer over time — and save you from needing new pants.

Four Common Changes

You Start Losing Muscle

Your Hormones Fluctuate

You’re Eating More

You’re Moving Less