Preterm births are the leading cause of infant death and long-term disabilities throughout the world.

New research by Sven Cnattingius, M.D., Ph.D., of the Karolinska Institutet in Stockholm, and colleagues indicates that women who are obese or overweight are more likely to deliver premature children.

In a study from 1992 to 2010, a mother's body mass index (BMI) and gestational age of babies delivered were measured and compared among Swedish mothers.

BMI is calculated using a person's height and weight to indicate that individual's amount of body fat. Body fat, while necessary for insulation, proper development, and energy for the body, can be problematic if there is too much of it in the body. According to the World Health Organization, excess body fat creates health risks for developing heart disease, decreased lung capacity, organ failure, and joint issues. Normal levels of body fat range from 18 to 25. Overweight people have BMIs of 25 to 30, while obese people have BMIs from 30 upward.

The researchers also identified the number of weeks preterm that a child could be delivered as extremely early at 22 weeks, very early at 28 weeks, and moderately early at 32 weeks. The Centers for Disease Control and Prevention (CDC) considers any birth before the 37th, or last, week of pregnancy to be preterm. However, in this study, the researchers have defined certain ranges of risk, with extremely early being the highest risk time to give birth and moderately early being less risky.

Preterm birth, no matter when it occurs, brings about the likelihood of developmental issues for that child. The CDC reports that many organ systems, like the lungs and brain, need every week of pregnancy in order to develop properly. Each year, one in nine children are born prematurely, with higher risks of death and disability the earlier a child is born. In 2008, 35 percent of children born prematurely died; this is more than any other single cause of infant death.

Extremely preterm delivery rates among women with normal BMIs was 0.17 percent, 0.27 percent for overweight women, and 0.35 to 0.52 percent for women who were obese. The most obese women, with BMIs over 40, had a 0.3 percent higher risk than anyone else of delivering a child preterm. A clear trend regarding obesity and the likelihood to deliver a very early preterm birth, around the 22nd week, has been delineated. Risk of delivering preterm increases as BMI increases. The women with very high BMIs had preterm deliveries associated with health issues due to their weight.

Often, overweight people have health issues that add up. Once one's body fat reaches a certain level, often seen in overweight and obese people, the fat tends to interfere with normal organ function. It begins to store itself everywhere, the most dangerous location being atop organs. If fats begin to deposit over the lungs, heart, liver, or kidneys, they will diminish their function. Organ function is important to everyday life, not to mention for pregnant women, whose bodies are keeping themselves as well as another living being alive.

Dr. Frederick Licciardi, M.D., of the NYU Fertility Center at NYU School of Medicine said, "...some doctors feel that morbid obesity creates an inflammatory condition throughout the body and this, in turn, could negatively effect the growth and development of the fetus." Obese people tend to have a low level of ongoing bodily inflammation. The body registers fat deposits as dangerous and disease-causing, and ultimately wants to destroy them. Of course, an immune system cannot deplete fat stores, so the immune cells are constantly acting in an effort to get rid of the fat invasion to no avail. As a result, the immune system may begin to act on the fetus developing in an obese woman, which will cause a preterm delivery, as the inflammatory molecules can no longer provide the fetus with a space for development.

While the percentage of extremely early deliveries of babies may sound small — 0.17 to 0.52 percent — this is still problematic. The health risks of an obese person, topped by the health risks that can occur during pregnancy, create an unfriendly environment for the fetus. Licciardi cited that people suffering from obesity are likely to develop diabetes and hypertension. "These diseases can also lead to poor fetal and placental development, leading to a smaller baby," he said.

Early deliveries cause many ongoing health and developmental issues such as cerebral palsy, mental disabilities, respiratory issues, vision and hearing loss, and digestive upsets. Preterm births are the leading cause of death among infants in the world, and given that 28 percent of Swedish women are obese and 1.4 billion adults worldwide were obese in 2008, this report gives all of us some serious data to contend with.

Source: Cnattingiius S, Villamor E, Johanson S, et al. Maternal Obesity and Risk of Preterm Delivery. JAMA. 2013.