Preeclampsia -- out-of-control hypertension in pregnancy -- is the number-one cause of maternal death around the world. But another disease is taking a toll on mothers and babies, especially in the United States -- obesity.
"The number-one health condition that affects pregnancy is obesity," said Dr. Katharine Wenstrom, professor of obstetrics and gynecology at Women and Infants Hospital at Brown University in Rhode Island. "It changes everything with an increased risk for all pregnancy complications."
More than half of all women in the U.S. are overweight or obese when they become pregnant, and most go on to gain more than the recommended amount during pregnancy, according to Kaiser Permanente.
Doctors now know that fat cells are not inert and can produce inflammation. For pregnant women, that inflammation affects the placenta, according to Wenstrom. Obese women have smaller babies, more pre-term labor, airway problems and chances of serious complications during a C-section.
"The chance of death is higher and it's a real risk," she said.
"Most women do just fine," said Dr. Maurice Druzin, chief of obstetrics and gynecology at Lucile Packard Children's Hospital at Stanford University.
But social changes such as women delaying childbirth are making pregnancy more complicated. Advanced reproductive technologies have allowed women "who are not supposed to get pregnant" to have children, said Druzin.
"If you left it to nature, they would not get pregnant at this age," he said. "There are big ramifications of infertility with more multiples. ... Women who are older tend to have more chronic medical illnesses like hypertension and diabetes and are at more risk of getting gestational diabetes and preeclampsia."
A 2010 report from Amnesty International, "Deadly Delivery: The Maternal Health Crisis in the USA," said "near misses" are increasing by 25 percent since 1998.
Of the 1.7 million American women who get pregnant, one third have medical complications, most of them among minority populations, according to the report.
Wenstrom said the report paints a somewhat distorted picture of prenatal health care in the United States, but several common conditions can make pregnancy more perilous.
Obesity is defined as a higher-than-average and unhealthy body mass index (BMI). Adults with a BMI between 25 and 30 are considered overweight. A person with a BMI greater than 30 is obese, and anyone more than 100 pounds overweight and has a BMI of 40 or greater is morbidly obese, according to the National Institutes of Health.
"Obesity rates are spectacularly rising," said Druzin. "And it's not only the rate that's rising, but the type of obesity -- morbid obesity. What we are seeing today is astonishing -- huge people."
Those who are obese are more predisposed to hypertension and diabetes, have more dysfunctional labors and, if they need surgery (a Caesarian), are more at risk because their airways can be compromised as tissues swell more readily.
"You cannot get the same quality of image in a sonogram," said Druzin. "It's challenging, and there is pretty good data showing that no matter how good the machine or skill of the doctor is, you may not get adequate imaging. The chance of missing something is there."