Exposure to smoking before and after birth is a risk factor for Sudden Infant Death Syndrome (SIDS), according to a new study. The number-one risk factor is still tummy sleeping, the study concluded.

Other risk factors include bed-sharing with parents and having objects in the crib, HealthDay reports.

The rate of SIDS has dropped by more than 50 percent in the United States since health officials began a campaign in 1994 to encourage parents to put babies to sleep on their backs, the article notes.

The study analyzed 568 SIDS deaths in San Diego between 1991 and 2008. The researchers found the percentage of babies who died of SIDS who were found on their stomachs dropped from 84 percent to 30 percent during that time.

“Exposure to cigarette smoke, either when the baby is in utero or after the baby has been born, is a very important risk factor for SIDS,” said researcher Dr. Henry Krous, Director of the San Diego SIDS/Sudden Unexplained Death in Childhood Research Project at Rady Children’s Hospital. He noted the study’s most important implication was that parents should avoid multiple risk factors.

The study appears in Pediatrics.