Studies Link Mothers’ Obesity to Babies’ Neural Tube Defects
Adding to the growing concern over the hazards of obesity to pregnant women and their babies, two new studies have found that women who are obese at the time of conception are at least twice as likely to have children with neural tube defects, such as spina bifida, as women of normal weight.
Among the heaviest women, the risk rose to four times normal, according to the reports in today’s Journal of the American Medical Assn. The studies represent the first time a specific birth defect has been definitively linked to a mother’s weight, according to Dr. James Mills of the National Institute of Child Health and Human Development.
Researchers from the California Birth Defects Monitoring Program and Boston University found, furthermore, that folic acid supplements, which normally protect against neural tube defects, are of no benefit in women who are at least 20% overweight.
Neural tube defects are the most common and severe congenital malformations, affecting about six children in every 10,000 in California. Spina bifida, which accounts for more than half of the defects, is an incomplete closure of the spinal column that usually results in paralysis of the legs.
“It was not too surprising to find that there are complications during childbirth in obese women, but it is very surprising to find a specific anatomical defect” in the infant, said Dr. Richard B. Johnston of the March of Dimes Birth Defect Foundation.
Pre-pregnancy counseling has historically focused on the health hazards of underweight mothers, such as premature, underweight infants, he said. “These two studies tell us we also need to focus on obese women and try to get their weight down to a level that is closer to ideal.”
“This expands the very short list of metabolic factors that cause birth defects,” Mills said. The other two are diabetes and folic acid deficiencies. Otherwise, most birth defects are genetic in origin.
Experts are at a loss to explain how maternal obesity causes the defects, however. “We do know that a link exists,” said epidemiologist Gary M. Shaw of the California program. “Right now we don’t know why. It could be something nutritional that we were unable to measure; it could be something metabolic in how women handle nutrient intake.”
Support for the speculation that the birth defects are nutritional in origin comes from the finding that folic acid apparently has no protective effect in obese women. Previous studies have shown that folic acid supplements can prevent 50% to 70% of cases of neural tube defects. Apparently, normal dietary levels of folic acid are not adequate in the obese women, or their metabolism has changed so that they cannot use it effectively, Shaw suggested.
Two previous studies have suggested that obese women metabolize some nutrients differently than lean women, according to Dr. Robert L. Goldenberg of the University of Alabama at Birmingham. In separate studies, Goldenberg has found that supplements of zinc or aspirin can stimulate fetal growth, reducing the incidence of low-birth-weight babies. Neither supplement is effective in obese women, suggesting again that there is a fundamental difference in their metabolic processes.
In the first of the studies reported today, Shaw and his colleagues studied 538 infants with neural tube defects born in California between February 1989 and May 1991. They compared them to 539 healthy infants born in about the same period. Mothers were interviewed after the births and asked about their general health, food consumption, education, drug use and a variety of other health and lifestyle factors.
To eliminate height as a variable, the team grouped women by body mass index, in which their weight at the time of conception (in kilograms) is divided by the square of their height (in meters). The researchers used a body mass index of 29 as a cutoff point for obesity: That corresponds to a 5-foot 6-inch woman weighing more than 180 pounds or a 5-foot woman weighing more than 149, Shaw said.
They concluded that women with a body mass index of more than 29 were 1.9 times as likely to have a baby with a neural tube defect.
Dr. Martha Werler and her colleagues at Boston University used slightly different techniques to produce virtually identical results. They studied 604 infants born in Boston, Philadelphia and southeastern Ontario, Canada, with neural tube defects and compared them with 1,658 infants born with other malformations, such as chromosomal abnormalities. They interviewed the mothers in the same manner, but did not consider the mother’s height.
They found that the risk of bearing a child with a neural tube defect rose as weight increased. Women weighing 176 to 196 pounds at the time of conception had 1.9 times the risk of women weighing 110 to 130 pounds, while those weighing more than 242 pounds had four times the risk.
In a separate study published today in the journal Pediatrics, researchers also found a new link between smoking and birth defects. A team from Emory University found that pregnant women who smoke are 50% more likely to have mentally retarded children. The study was the first to connect smoking with retardation, said epidemiologist Carolyn D. Drews of Emory.
The Emory team compared 221 mentally retarded children in the Atlanta area with 400 healthy controls, using the same interview techniques as the obesity researchers.
