Children born to mothers who drank and smoked beyond the first three months of pregnancy have 12-fold increased risk for sudden infant death syndrome (SIDS), says a new study.

SIDS is the sudden, unexplained, death of an infant under one year of age. Many studies have shown that the risk of SIDS is increased by maternal smoking during pregnancy.

Some studies have also found that prenatal alcohol exposure, particularly from heavy drinking during pregnancy, can increase SIDS risk.

The findings, published in the journal The Lancet, provide a look at how SIDS risk is influenced by the timing and amount of prenatal exposure to tobacco and alcohol.

"Our findings suggest that combined exposures to alcohol and tobacco have a synergistic effect on SIDS risk, given that dual exposure was associated with substantially higher risk than either exposure alone," said said first author Amy J Elliot from Avera Health Centre for Pediatric and Community in US.

For the findings, researchers followed the outcomes of nearly 12,000 pregnancies among women from two residential areas in Cape Town, South Africa; and five sites in the US.

The study sites were selected for their high rates of prenatal alcohol use and SIDS, and to include populations where the ethnic and socioeconomic disparities in SIDS remains understudied.

The researchers determined one-year outcomes for about 94 per cent of the pregnancies.

They found that 66 infants died during that time, including 28 SIDS deaths and 38 deaths from known causes.

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In addition to the almost 12-fold increased SIDS risk from combined smoking and drinking beyond the first trimester of pregnancy, they determined that the risk of SIDS was increased five-fold in infants whose mothers reported they continued smoking beyond the first trimester, and four-fold in infants whose mothers reported they continued drinking beyond the first trimester.

According to the researchers, these risks were in comparison to infants who were either not exposed to tobacco or alcohol during gestation or whose mothers quit tobacco or alcohol use by the end of the first trimester. (IANS)