1 September 1998

University of Queensland researchers have discovered a possible mechanism predisposing babies of mothers who smoke while pregnant to Sudden Infant Death Syndrome (SIDS).

For her PhD with the University's Perinatal Research Centre within the Obstetrics and Gynaecology Department, Carol Browne conducted four standard tests of autonomic nervous system functions of 56 newborn babies at the Royal Women's Hospital.

Around half the babies were born to mothers who had smoked while pregnant. Thirty-three of the babies were tested again at three months of age.

She found a weaker autonomic nervous system response in newborns of mothers who smoked while pregnant in two of the four tests.

The autonomic nervous system drives the body's involuntary functions such as the ongoing rhythm of breathing, heart rate and certain bowel activities.

The thesis, supervised by Professor Paul Colditz, represents the most comprehensive testing of newborn autonomic nervous system responses in a bid to establish a link between mothers who smoke while pregnant and a predisposition to SIDS.

Professor Colditz said while it was known babies of mothers who had smoked while pregnant were more vulnerable to SIDS, Ms Browne's study was the first to pinpoint the physiological reason for this vulnerability.

"The body of evidence suggests SIDS results after some dysfunction in a baby's cardiac control or respiration triggered by an environmental reduction in oxygen," he said.

"Ms Browne's study shows that babies born of mothers who smoked while pregnant have differences in their autonomic nervous systems controlling breathing and heart rate."

Professor Colditz said the study results reinforced the message to pregnant women not to smoke. He said it had been estimated that the number of deaths from SIDS would be reduced by at least 40 percent if pregnant women ceased smoking while pregnant.

"At present, around a third of pregnant women smoke and babies of mothers who smoke while pregnant have three times the risk of dying from SIDS," he said.

Funded by $17,000 from the South Australian SIDS Research Foundation and $5000 from the Community Gaming Benefit Fund, Ms Browne, who works as a Centre research officer, is continuing her work thanks to a Lions Medical Research Foundation Fellowship.

In one test, newborns were exposed to a slightly reduced oxygen content in the air for short periods. The level was cut from the normal 21 percent oxygen to around 15 percent to mimic the conditions of being covered by a blanket or other covering - a prime risk factor for SIDS.

Ms Browne found that after three minutes of reduced oxygen, there was less oxygen in the blood of infants of mothers who smoked while pregnant than in the infants of mothers who didn't.

"This result could mean that infants of mothers who smoked while pregnant are more susceptible to situations of reduced oxygen such as occurs when the head is accidentally covered by the bed covers," Ms Browne said.

In the second test of autonomic nervous system responses, blood pressure and heart rate were measured after the infants were tilted from the horizontal to the upright position.

Ms Browne said infants of mothers who had smoked while pregnant had an increased basal blood pressure and less mature response to the test compared to the babies of mothers who had not smoked while pregnant. These babies' response more closely resembled that expected in a normal, healthy adult.

"This result could indicate a delayed maturation in infants of mothers who smoked while pregnant, making them again more susceptible to SIDS. We already know nicotine can alter the development of various areas of that baby's brain. The brains and bodies of these babies are consistently smaller than others," she said.

SIDS is the most common cause of death among Australian infants aged between one month and one year. It results in the death of one baby per 1000 Australian births and more than 40 babies die from SIDS in Queensland each year.

Professor Colditz is currently collaborating with researchers from the University's Indigenous Health Program on two new SIDS-related projects funded by Queensland Health.

The first aims to identify the barriers to getting the public health message about SIDS into the indigenous community. The second aims to develop resources to assist indigenous parents to make appropriate decisions about their children's health when presented with the signs and symptoms of illnesses.

For more information, contact Professor Colditz (telephone 07 3253 1760) or Ms Browne (telephone 07 3253 1764).