If the kilos are piling up for no apparent reasons, here are five conditions that might explain why your jeans are getting so snug.
Got insomnia? Sleep apnea? Restless legs syndrome? "Sleep deprivation from any cause can contribute to weight gain," says Dr Lisa Neff, an endocrinologist at the Center for Lifestyle Medicine at Northwestern Medicine in Chicago.
Lack of sleep interferes with hormones that signal hunger and fullness, so people who don't snooze enough are more apt to overeat. Plus, a recent study found people who sleep less find eating more enjoyable—which means you're more likely to binge on guilty pleasures.
Of course, your activity level also tends to dip when you're tired, so you probably won't burn off those extra kilojoules, either.
People who suffer from depression often gain weight for a few different reasons. "Some people find themselves eating emotionally," Neff says.
"Depression can also affect physical activity. And many antidepressant medications cause weight gain as well."
Antidepressants aren't the only meds that can have weight gain as a sneaky side effect. There are drugs in just about every class that have the potential to impact your weight, says endocrinologist Dr Amy Rothberg. Some common culprits include insulin, beta blockers and contraceptives.
When patients come in with sudden unexplained weight gain, Rothberg always checks to see if they've recently started a new medication.
Any condition that causes chronic pain, including fibromyalgia, arthritis and sciatica, can wreak havoc on your waistline.
People who are in pain tend not to be very active or sleep very well, says Neff.
Pain medication, especially drugs that are commonly prescribed for nerve pain, may also contribute to weight gain.
It's rare, but this endocrine disorder caused by an excess of the hormone cortisol can cause your weight to spike suddenly. It also tends to change the way your weight is distributed.
"With Cushing's, the face becomes very round," Rothberg says.
You'll probably also have a big belly despite having thin limbs and muscle atrophy. A physical exam can help your doc spot the hallmark signs.
It's more a stage of life than a condition, but gaining a few kilos during menopause is relatively common. "Women often come in to see me with what they perceive as a surprising or sudden weight gain," Neff says. "They find what they've done before to keep their weight in check is no longer working, and they've gained 10 kilos despite their best efforts."
Hormones are only partially at fault - hormonal changes can cause hot flashes that interrupt sleep, which in turn impacts weight.
But much of the blame goes to plain and simple ageing, which can slow your metabolism.