The research, which followed more than one million UK women, confirms a link between obesity and the risk of venous thromboembolism (VTE) -- blood clots in the veins, usually in the legs. If one of those clots breaks free and travels to the lungs, causing what's called a pulmonary embolism, it can prove fatal.
The findings also show that heavier women are more likely to end up needing surgery -- which is itself a major risk for VTE.
That's "not entirely unexpected," since obesity raises the risk of some medical conditions that could lead to surgery, said lead researcher Lianne Parkin, of the University of Otago in New Zealand.
"But as far as we know, our study is the first to directly examine the relationship between being overweight or obese and the likelihood of having an operation," Parkin told Reuters Health in an email.
The researchers found that for every 1,000 normal-weight women who had inpatient surgery over six years, about five developed a clot in a deep vein or a pulmonary embolism within 12 weeks of the operation.
Among overweight and obese women, that rate was seven per 1,000.
The risk of suffering a clot without surgery was far lower, but still relatively higher among heavier women.
Of normal-weight women, 0.1 out of every 1,000 developed a VTE during any 12-week period in which no surgery was done. The rate was 0.2 for every 1,000 overweight or obese women.
Overall, Parkin said, the risk of clots climbed in tandem with a woman's weight. "That suggests that the loss of even small amounts of weight is likely to be beneficial (in terms of reducing VTE risk) for women who are overweight or obese," she said.
The findings, which appear in the journal Circulation, are based on more than 1.1 million UK women who were 56 years old, on average, at the study's start. The researchers used hospital records and death certificates to track cases of VTE over six years.
During that time, 6,438 women were hospitalized for, or died from, a VTE -- with almost 1,900 forming a clot within 12 weeks of an operation. (That was out of more than 641,000 women who had at least one operation during the study period.)
Women who were overweight or obese were 22 per cent more likely to need inpatient surgery versus their thinner peers.
That means more overweight women will face the chance of a surgery-related VTE, and their risk with any given surgery will be relatively higher compared with thin women.
According to Parkin's team, their figures probably underestimate the actual number of women who developed a VTE -- since clots in the leg veins may be detected and treated by a primary care doctor.
Those clots are almost always diagnosed because of symptoms, like pain in the calf, swelling in the ankle and foot and warmth over the affected area.
Treatment can include medication to keep a clot from growing or prevent new ones. Wearing compression stockings around the lower leg can also help prevent new clots.
According to Parkin, the best way for an overweight woman to cut the risk of a non-surgery-related VTE is to lose some weight. And that would come with "many other important health benefits," she pointed out.
"In addition to weight loss, though," Parkin added, "it is important to increase physical activity. Immobility is a risk factor for VTE, and overweight and obese people are often less physically active."
If you're facing surgery and have enough advance warning of it, shedding some weight is, again, a good idea, Parkin noted. In addition, you can ask your doctor what will be done to minimize any risk of post-surgery VTE.
There are different recommendations on how to help prevent surgery-related VTE, including the use of "blood-thinning" drugs. And those vary based on the type and duration of the surgery, Parkin said.