Epidemiological Factors

While the cause of SIDS remains elusive, multiple studies have documented consistent epidemiological factors associated with higher SIDS risks in some groups of infants. Risk factor categories include maternal and prenatal, neonatal (newborn), postneonatal, geographic, and race/ethnicity groupings.

Maternal and prenatal risk factors constitute a lengthy list of biological and environmental conditions. These include shorter interpregnancy interval, increased placental weight, low socioeconomic status, nutritional deficiency, anemia, urinary tract infection, intrauterine hypoxia (oxygen deficiency), fetal growth retardation, smoking, drug exposure, poor prenatal care, young age, lower education, and increased number of pregnancies. Several studies have identified maternal smoking as a significant risk factor. The National Institute of Child Health and Human Development (NICHD) conducted a large study in the United States of 757 SIDS cases with two matched control groups. Seventy percent of the SIDS mothers in this study smoked. When compared with the control groups, the risk for infants of mothers who smoked is doubled and progressively increases as the number of cigarettes smoked per day increases. These infants also die at younger ages. Constriction of blood vessels leading to chronically diminished oxygen delivery to fetal tissues is thought to be the mechanism by which smoking increases the risk of SIDS.

Neonatal risk factors include poor growth, asphyxia (inadequate oxygen delivery to body tissues), prematurity, and low birthweight. As the gestational age decreases, the relative risk of SIDS increases. This is also true of birthweight. The incidence of SIDS in preterm infants whose birthweight is greater than 1,500 grams (3 pounds, 5 ounces) is about 8 per 1,000 live births, compared to preterm infants with birth-weights less than 1,500 grams, where the risk rises to 10 per 1,000 live births. Postnatally, male sex, age (two to four months), bottle feeding, overheating, smoking exposure, soft bedding materials, no pacifier use, and prone sleeping position have been identified as significant factors that independently increase the risk of SIDS.

Geographic and race/ethnicity factors play an additional role in increasing the relative risks. SIDS rates increase during cold weather months, in economically poor countries, and in infants of black race or Native-American ethnicity. Worldwide, groups such as Gypsy, Maori, Hawaiian, and Filipino also have increased SIDS rates.

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