The sudden death of an apparently healthy infant that remains unexplained after all known possible causes have been ruled out through autopsy, death scene investigation, and review of the medical history.
SIDS is the leading cause of death in infants between 1 week and 1 year of age, with an approximate rate of 2 per 1,000 live births; 6000–7000 babies die of SIDS every year in the U.S. The peak age is 2–4 months and the most deaths occur during the colder months (October to April in the northern hemisphere). The case definition specifically excludes death due to drugs or poisons, apnea, respiratory infection, suffocation, aspiration of vomitus, choking, accidental strangulation, or child abuse. Most victims appear healthy before death, which occurs rapidly, usually during sleep. SIDS strikes families of all races and socioeconomic levels. It is slightly more common in males, and the second child is more susceptible than the first. Some theories suggest a congenital or developmental defect, but the phenomenon does not show familial clustering. Statistical studies have identified certain risk factors, among them maternal smoking before and after birth, inadequate prenatal care, low birth weight, young maternal age, and maternal hard drug use. Some but not all studies have suggested that breast-feeding slightly reduces the risk. Gastric infection with Helicobacter pylori has been speculatively implicated in some cases. The most important risk factor identified to date is sleeping in the prone position. Sleeping on the side is less dangerous than sleeping prone but more dangerous than sleeping supine. The reason for these differences is unknown, but the incidence of SIDS has declined markedly since 1992, when the American Academy of Pediatrics first recommended that healthy infants be placed on their backs for sleep. For infants with gastroesophageal reflux, swallowing dysfunction, or unilateral vocal cord paralysis, the prone position may be preferred. For healthy infants, the supine position does not increase the risk of vomiting and aspiration. Current medical practice emphasizes reduction of risk through avoidance of the prone sleeping position and of maternal smoking, and education, counseling, and emotional support of the parents of victims.
Reference: Stedman's Medical Dictionary