Combined drinking and smoking in the first trimester of pregnancy can lead to a 12-fold increased risk of Sudden Infant Death Syndrome (SIDS), according to a new study supported by the US National Institutes of Health.

SIDS is the sudden, unexplained, death of an infant under one year of age. A report from the Safe Passage Study published in EclinicalMedicine, an online journal published by The Lancet, looked at how SIDS is influenced by the timing and amount of prenatal exposure to tobacco and alcohol.

A multicentre team of scientists from throughout the US and in South Africa formed the Prenatal Alcohol in SIDS and Stillbirth (PASS) Network. From 2007 until 2015, PASS Network 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, including two American Indian Reservations in South Dakota and North Dakota. 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% of the pregnancies. They found that 66 infants died during that time, including 28 SIDS deaths and 38 deaths from known causes. 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.

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.

Screening for substance use early in pregnancy needed

Author Hannah C. Kinney, Department of Pathology at Boston Children's Hospital and Harvard School of Medicine, said: "The Safe Passage Study provides important new information about the role of dual exposures to prenatal smoking and drinking as risk factors for SIDS. Our findings support the current recommendation of the US Centers for Disease Control and Prevention and the World Health Organization that women not drink or smoke during pregnancy, and emphasises the significance of dual exposure, which provides the greatest risk for infant mortality."

In a joint statement, the leaders of the NIH Institutes that provide primary funding for the Safe Passage Study said: "These findings provide still more evidence of the vital importance of the early prenatal environment to healthy postnatal outcomes. Insofar as many women quit drinking and smoking only after they learn that they are pregnant, this study argues strongly for screening for substance use early in pregnancy and intervening as soon as possible. It also calls for stronger public health messaging regarding the dangers of drinking and smoking during pregnancy, and among women who plan to become pregnant."