Low Birth-Weight, High Blood Pressure Link Disputed

New analysis refutes widely held belief

THURSDAY, Aug. 29, 2002 (HealthDayNews) -- The widely held belief that low birth-weight babies may be prone to high blood pressure as adults is wrong, a new analysis of previous studies contends.

After reviewing 55 studies that had reported such a link, Australian researchers found that the larger studies revealed weaker cause-and-effect associations.

"Strong claims for an inverse association between birth weight and blood pressure have been made, and the relationship has been considered to provide some of the strongest support for the 'fetal-origins hypothesis,' " says Rachel Huxley, a researcher at the Institute for International Health in Sydney.

The hypothesis contends that many adult diseases have their start in the womb.

But it doesn't hold up for birth weight and blood pressure, says Huxley, whose findings appear in the Aug. 31 issue of The Lancet.

"After carefully reviewing all of the published studies, there is no good evidence to suggest that there is a causal association between birth weight and blood pressure," she says. "It would therefore be prudent to treat with caution any other claims of associations between size at birth with later health outcomes."

Experts had estimated that every 2.2 pound increase in birth weight reduces systolic blood pressure (the higher number of the two blood-pressure readings) by two to four millimeters of mercury later in life.

Low birth weight is defined as less than 5.5 pounds, or 2,500 grams, at birth. The condition affects about one in 14 babies born in the United States each year, according to a report earlier this year in the Journal of the American Medical Association.

Why were the earlier studies that found an association between low birth-weight and later blood pressure problems off the mark?

"No allowance was made for the size of the contributing studies," Huxley says. "Previous reviews had combined the results from all studies, making no allowance for whether the study population consisted of only a few hundred individuals or tens of thousands of people."

And twin studies were excluded from the earlier reviews, Huxley adds. They're valuable, she says, because studies of twins are less prone to "confounding factors," differences that can affect outcomes or results.

Health experts offer praise for the Australian study, but say it doesn't mean mothers-to-be shouldn't pay close attention to habits that help reduce the risk of a low birth-weight baby.

"I think this review is pretty interesting and well done," says Dr. F. Ralph Dauterive, chief of obstetrics at the Ochsner Clinic Foundation in Baton Rouge, La. "You have to be careful drawing conclusions from small studies and this [new review of 55 studies] is a good example of why.

"Parents need to be concerned with low birth-weight for many other reasons," he adds. A low birth-weight baby is more likely to have health problems, such as an abnormal body temperature, feeding problems, infection, and, if also premature, underdeveloped lungs and breathing problems.

To help prevent low birth-weight babies, experts recommend that pregnant women have regular checkups, eat a balanced diet containing sufficient calories and vitamins and minerals, gain a healthy amount of weight (usually 25 to 35 pounds) and avoid tobacco and alcohol.

"I worry more about the development of low birth-weight babies, more so than about their blood pressure later on," Dauterive says.

Another expert, Kevin Blake of the University of Western Australia, Subiaco, says an adult's current weight has a more substantial effect on raising systolic pressure than low birth weight. He suggests that study of birth weight is "a very crude marker of growth in the womb."

In a recent study, he measured fetal growth by using ultrasound to measure the thigh bones and found that the longer the thigh bone, the lower the systolic blood pressure later in life. Tracking fetal growth provides more opportunity for medical intervention, he adds.

"My study is saying that a shortened thigh bone in the fetus relatively early in pregnancy may be an indicator that something is not right with the environment in the womb and that the mother's behavior and known risk factors [smoking, lack of exercise, poor diet] need close scrutiny to help the fetus later in its development. In other words, to do something when something can actually be done," he says.

Parents of low birth-weight babies shouldn't worry their children will grow up to have high blood pressure, Huxley concludes.

"Factors such as current body weight [when they are older], diet and exercise remain among the most important, most easily modified determinants of blood pressure," she says.

What To Do

For information on how parents' behavior affects the development of low birth-weight babies, visit the Center for the Advancement of Health. For information on having a healthy pregnancy, go to the National Women's Health Information Center.

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