December 1 (HealthDay News) -- Premature infants often have lower-than-normal blood pressure that persists during the first six months of life and may be one reason these infants are more prone to sudden infant death syndrome (SIDS), Australian researchers suggest.

One theory of the cause of SIDS is a profound drop in blood pressure during sleep, from which the infant cannot recover.

"Premature babies are at increased risk for SIDS, and we investigated a possible mechanism of how these babies could be at increased risk," explained lead researcher Rosemary S.C. Horne, of the Ritchie Centre for Baby Health Research at the Monash Institute of Medical Research at Monash University in Melbourne.

For the study, Horne's team monitored the heart rates of 25 premature infants, comparing them with 20 infants born at term. The researchers looked at the infants' blood pressure at two to four weeks, two to three months, and five to six months.

They found that preterm infants had lower blood pressure during sleep compared with normal-term infants. This was true during all the periods the blood pressure was measured.

Systolic blood pressures in preterm infants were 11 mm Hg to 14 mm Hg lower than in full-term infants, Horne said. "This may be clinically significant," she noted.

"We hypothesize that if blood pressure is already low in preterm babies, then if there was an event during sleep that made it fall even further they may be at risk of a profound drop in blood pressure," Horne said.

"In addition, this may explain the risk of preterm babies having an increased risk of SIDS and it may also indicate that these preterm babies, although they appear well and healthy, may have long-term alterations in their cardiovascular control," she said.

The findings were published in the December issue of Pediatrics.

Dr. Warren G. Guntheroth, a professor of pediatrics at the University of Washington School of Medicine in Seattle, doesn't think low blood pressure is a cause of SIDS. He believes the trouble lies with breathing problems.

"I do not believe there is enough evidence that control of blood pressure is inadequate just because the systolic pressure in preemies is lower than average," Guntheroth said. "This may well be because they are small."

Preemies do have pulmonary problems, of course, and it's likely that this accounts for an increase in SIDS, Guntheroth said. "The pulmonary problems may induce a degree of low oxygen that can increase the frequency of apnea with bradycardia, but that is a very basic response that does not indicate an immaturity of control of the cardiovascular system," he said.

Another SIDS expert also said it's not known whether low blood pressure increases the risk of SIDS.

"The unresolved question is whether these differences in blood pressure have any relevance for the increased risk for SIDS that preterms have -- that's unclear," said Dr. Carl E. Hunt, an adjunct professor of pediatrics at the Uniformed Services University of the Health Sciences, in Bethesda, Md.

"The extent to which the inability to recover normal blood pressure is a precipitating event for SIDS, we have no idea," said Hunt, who's also a director at the American SIDS Institute. "It's difficult to prove and it has not been proven."

SOURCES: Rosemary S.C. Horne, Ph.D., Ritchie Centre for Baby Health Research, Monash Institute of Medical Research, Monash University, Melbourne, Victoria, Australia; Warren G. Guntheroth, M.D., professor, pediatrics, University of Washington School of Medicine, Seattle; Carl E. Hunt, M.D., adjunct professor, pediatrics, Uniformed Services University of the Health Sciences, Bethesda, Md., and member, board of directors, American SIDS Institute; December 2008 Pediatrics