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GALVESTON -- Infants from twin births have more than twice the risk of sudden infant death syndrome (SIDS) compared to babies of single pregnancies, a national study examining birth and death records of twins confirmed. Researchers from the University of Texas Medical Branch at Galveston also found an increased risk of a surviving twin dying of SIDS once his twin dies of the syndrome.

Michael H. Malloy, M.D., professor in the University of Texas Medical Branch at Galveston's Division of Neonatology conducted the study with co-author Daniel Freeman Jr., Ph.D., director of UTMB's Office of Biostatistics. Malloy presented the data last month at the annual meeting of the Pediatric Academic Societies in Washington, D.C.

The study, using national birth and death records from 1987, found 2.76 SIDS deaths per 1,000 live twin births -- more than double the rate of 1.2 SIDS deaths per 1,000 live births of single pregnancies.

Of the 66,276 fraternal and identical twins in Malloy's study, 183 were identified as dying of SIDS, including three pairs of twins. Cases of both twins dying were rare. But the risk of a surviving twin dying of SIDS given the death of its co-twin was considerably increased above the risk of either twin dying of the syndrome.

"If one twin dies of SIDS, the risk may be from six- to twelvefold greater that the other twin will die," Malloy said. "But the absolute number of such events occurring is still very low."

Malloy's study may also dispel a myth that twins die of SIDS on the same day. At least in 1987, none of the three pairs of twins dying of SIDS died on the same day.

"There is a lot of mythology out there about twins and SIDS," Malloy said. "This is an area that has not been researched a great deal."

SIDS is the leading cause of mortality after infants' first four weeks of life, with 4,073 deaths in the United States attributed to the syndrome in 1994. While the underlying mechanism for SIDS remains unknown, epidemiological studies have linked it with several factors, including the association with twin births.

"The increased risk of SIDS among twin births is particularly interesting because it brings forward the issue of trying to understand where environmental contributions causing SIDS may trail off and hereditary contributions to the cause of SIDS may begin," Malloy said.

SIDS has been linked to such environmental factors as infants sleeping on their stomachs, soft bedding, seasonal conditions, low birth weight and exposure to cigarette smoke.

Malloy said the increased risk of a second twin dying if its sibling dies of SIDS may simply show that both were exposed to the same environmental factors, or it may show that both were predisposed to the syndrome genetically.

"There's more we need to study," he said. "It's likely that a great deal of the risk can be ascribed to the environment, but we don't have enough information to rule out heredity."

Previous reports suggesting that twins have a higher incidence of SIDS looked at populations in smaller regions. The UTMB-funded study used a new computer program to identify twins using national vital statistics. Twins -- not identifiable as belonging to the same pair in birth or death records -- were matched using birth dates and locations, as well as demographic information about their parents.

"It gives a good look at what's happening across the United States, not just one specific area," Malloy said.

Malloy has applied for grants to study more recent years in order to determine if the declining rate of SIDS in the general population is mirrored in the twin population. That decline followed campaigns to inform parents of the importance of laying their children on the backs rather than their stomachs.

Story Source:

Adapted from materials provided by Johns Hopkins Children's Center.

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