An obese Massachusetts woman and her 8-month-old fetus died of complications 18 months after stomach-stapling surgery, an apparent first that is leading to warnings about the risks of pregnancy soon after the surgery.

The deaths raise concerns because most of the 110,000 people who have gastric, or stomach, bypass surgery each year in this country are women in their child-bearing years, say doctors at Brigham and Women’s Hospital in Boston who tried to save the mother and baby. They reported on the case in a letter in Thursday’s New England Journal of Medicine.

Brigham surgeon Dr. Edward E. Whang cared for the 41-year-old woman nearly two years ago, when she was brought to their emergency department after two days at another hospital, where the cause of sudden pain in her upper stomach was misdiagnosed.

“She was nearly dead,” said Whang, who noted the woman had the gastric bypass and prenatal care at other hospitals.

After tests and symptoms indicated a massive infection, Whang did emergency surgery. He found most of the woman’s small intestine had slid through a hernia, or tear, in an adjacent membrane, a defect sometimes left after the intestines are rearranged in the bypass operation. The hole choked off blood to the stretch of intestines, and the tissue turned gangrenous.

By then the fetus had died. Whang repaired the intestine, but the woman died within a few hours. She still weighed 440 pounds; her original weight was not available.

“This is a tragic case, but you need to look at the overall risk-benefit of the surgery,” said Dr. Harvey Sugerman, president of the American Society for Bariatric Surgery, an educational group for obesity surgeons.

Sugerman said three studies show extremely obese patients have death rates as much as four or five times lower if they undergo gastric bypass surgery, compared with those who don’t, and other research shows lower rates of pregnancy complications after the weight-loss surgery.

Sugerman estimated more than 110,000 gastric bypasses will be done in the United States this year. Complications strike as many as 1 in 5 patients, and it is believed that for every 200 patients, 1 to 4 will die.

Sugerman and other doctors said they know of no other mother-baby deaths after the operation.

But Whang said about 85 percent gastric bypasses are performed on women, most of childbearing age. He advised any pregnant patients with sudden pain to see a bypass expert quickly.

“Most doctors recommend contraceptives for two years after the surgery” to avoid pregnancy until the woman’s weight stabilizes, said Dr. Sattar Hadi, who runs the high-risk obesity clinic at Vanderbilt University’s Center for Human Nutrition.

Hadi said it’s unclear whether the Massachusetts woman’s hernia was due solely to the surgery “or the fetus pushing onto the intestines.”

Dr. Mark Tucker, director of bariatric surgery at the University of Medicine and Dentistry of New Jersey in Stratford, said such hernias are not uncommon and can happen up to five years after the surgery. Because fertility increases with major weight loss, he said, reports of pregnancies within a year of surgery are becoming more common.

In the standard type of gastric bypass surgery performed on this woman, a small pouch at the stomach’s top is separated from the rest using staples. Moving down the small intestine a few feet, the surgeon then cuts it in two, pulls the bottom segment up and attaches it to the new pouch. The dangling upper segment, which carries digestive juices, is then reattached to the lower segment.

Patients then lose weight because the new pouch holds a few ounces of food at a time, and some calories and nutrients are not absorbed because the food bypasses much of the stomach and small intestine. Patients must take protein and vitamin supplements to avoid malnutrition.

Since the start of 2003, three other Massachusetts patients have died after gastric bypass, and the state is developing guidelines to increase safety.

The federal Medicare program has paid for stomach bypass surgeries for a few years, but only in people with related medical conditions such as diabetes. It covered 7,801 operations in 2003, about double the number in 2001.

Dr. Steve Phurrough of the Centers for Medicare and Medicaid Services said the agency recently ruled obesity can be considered an illness on its own, so an advisory panel this fall is to recommend whether Medicare should cover the surgery in people without related conditions. He expects the panel will urge waiting until there’s more research on the surgery on the elderly.

“We have significant concerns about the risks,” Phurrough said. “People die from this procedure.”