Men born with a birth defect have a substantially increased risk of having a child with a birth defect, a large population study revealed today. Compared with other fathers, the risk was doubled.

The second-generation risk also appeared higher — at least for dissimilar birth defects — than for the offspring of mothers who had been born with birth defects.

Scientists at the U.S. National Institute of Environmental Health Sciences and Norway's University of Bergen reviewed Norwegian births since 1967. They compared 12,000 men who had been born with a recognized defect with nearly a half-million unaffected men. The scientists reported today in the Journal of the American Medical Association for Feb. 14 that the men had fathered 1,265 children and that:

• Twenty-one of these children (1.6 percent) had been born with the same defect as their father's. This is about seven times the risk of those same defects in the general population but is about the same risk as a previous study showed for the children of women with birth defects.

• However, the children of fathers with birth defects also had a higher risk of having different, unrelated birth defects. Forty-three children (3.4 percent) had defects that were not like their father's, compared to an expected number of 24 (1.9 percent). This result was in contrast to the findings of the previous women's study in which the children of women with defects appeared to have no increased risk of babies with defects different from the mother's.

• The total risk of birth defects was 5.1 percent among the offspring of fathers with defects, or twice the 2.1 percent risk of the offspring of other fathers. The risk was spread out across categories of defects, not concentrated in any one category.

"Five percent of children with birth defects is not a whole lot," Allen J. Wilcox, M.D., Ph.D., said, "but it still is more than double what we see in the children of unaffected fathers."

Dr. Wilcox is chief of epidemiology at the National Institute of Environmental Health Sciences, one of the U.S. National Institutes of Health.

"What surprised us," Dr. Wilcox continued, "is that the children of the affected fathers had a higher risk of all kinds of defects, not just the same defect as their father. In our earlier study of women with birth defects, this did not appear to be the case: The children seemed to have no special risk of birth defects except for the specific defect of the mother."

Dr. Wilcox' co-researchers are Rolv T. Lie, Ph.D., and Rolv Skjaerven, Ph.D., both professors at the University of Bergen. Their studies are based on the Medical Birth Registry of Norway which links the birth records of fathers, mothers and offspring through the Norwegian system of unique personal identification numbers.

The investigators grouped the birth defects recorded into 24 categories. Cleft lip, genitalia defects, limb defects and clubfoot (in which the foot is twisted out of position) were the four most common recurring defects that recurred in the offspring of affected fathers. In many cases, these defects can be surgically repaired.

In their report, the three said they had expected an excess in defects of the same type as the fathers because many birth defects are heritable. They said they had no explanation for the increased rate of dissimilar effects.

In the study, boys with birth defects had a lower-than-normal survival rate to age 20. Even if they survived to adulthood, they were 30 percent less likely to father a child than other men. This pattern of reduced reproduction (which the authors said presumably reflects social factors as well as biological) had also been seen among affected women.

The scientists said the higher death rates among babies with birth defects, as well as the reduced likelihood that the survivors will have children, reduces the impact of parents with defects on the next generation.

"We also need to put this into perspective," Dr. Wilcox said. "More than 95 percent of all babies with birth defects are born to parents who have no known birth defects themselves. Measles, a lack of folate in the diet, and heavy alcohol use are factors for some defects, but the causes of most birth defects, environmental as well as genetic, are not known. We have a lot to learn — still — about the cause and prevention of birth defects."

Dr. Wilcox can be reached at (919) 541-4660, and will be available for interviews beginning Monday, February 12.