New clues to origins of Downs' syndrome.

Older mothers are known to face a greater risk than younger women of giving birth to a baby with Down's syndrome, a genetic disorder characterized by mental retardation. But researchers have not been able to explain why. A new study of 158 families with children affected by Down's now supports one contending theory -- that older women are more likely to carry a Down's baby to term.

Human geneticist Stylianos E. Antonarakis of Johns Hopkins University in Baltimore headed an international team of 18 other researchers that determined the extra chromosome responsible for Down's is incorporated more often during the first of the human egg's two stages of meiotic development. Both younger and older women were equally likely to have a chromosome error in the first stage, regardless of their age, the researchers reported last week at a meeting of the American Pediatric Society and Society for Pediatric Research in New Orleans.

The team used DNA tags to trace the origin of the extra chromosome-21 in the Down's children. They found, as expected from their previous studies, that 95 percent of the extra chromosomes came from the mother and only 5 percent from the father. When they traced further, they found that three-fourths of all maternal chromosome errors resulted from the first stage of meiosis, the cell division process that halves the genetic content of eggs and sperm so that children don't get double the proper number of genes.

Although the results are difficult to interpret, Antonarakis says, "what was surprising was that we didn't see a difference in maternal age according to the stage of meiosis where the chromosome error occurs." If mothers of all ages have similar chromosome errors leading to Down's, he reasons, older mothers might have more Down's babies because their bodies fail to recognize an egg with an extra chromosome as abnormal. In other words, "It's possible that older mothers could carry a Down's baby to birth better," he told SCIENCE NEWS.

But Antonarakis cautions that his group did not study tissue from fetuses spontaneously aborted by the women, to see if younger mothers were more likely to miscarry a Down's child than older mothers. "It's possible that in aborted fetuses there is a stronger correlation between maternal age and the stage of meiosis leading to Down's," he says.

David H. Ledbetter, a geneticist from the Baylor College of Medicine in Houston, agrees that further studies will be needed to confirm that younger women are more likely to miscarry Down's babies. "The only way to directly test it would be to compare the ages of women who have children with Down's syndrome and those who spontaneously abort Down's syndrome fetuses," he says.