Apparently, due to fear of vomiting and aspiration, it became standard practice in many countries to place babies to sleep in the prone position. Without any scientific evidence to support the practice, so-called baby experts have been strongly recommending the prone infant sleeping position for decades. However, numerous studies from many countries, including the United States, clearly show that the prone sleeping position for infants is associated with the highest risk for SIDS, the supine sleeping position with the lowest risk, with the side-lying position conferring an intermediate risk for SIDS. The side sleeping position is associated with an increased SIDS risk because it is unstable, allowing infants to roll into the prone position. In 1992, the American Academy of Pediatrics (AAP) issued a policy statement recommending that healthy infants, when being put down for sleep, be positioned on their side or back. In 1996, in response to new evidence that the side position conferred an increased risk for SIDS, the AAP issued a new policy statement recommending that healthy infants should sleep on their backs. In the 4 years following the 1992 AAP statement, the proportion of infants sleeping prone in the U.S. declined from about 75% to about 20%, and the SIDS rate dropped approximately 40%.

How is sleeping position related to SIDS? Several investigators believe that it is related to developmental vulnerability, allowing a baby sleeping prone, with his face in contact with blankets or other porous bedding material, to rebreathe exhaled air, leading to asphyxia and suffocation. After about 6 months of age, when an infant can spontaneously change head, face, and body position and the cardiorespiratory control system is more mature, he is likely to be past the vulnerable period. Other possible contributing mechanisms include widespread alveolar collapse with hypoxemia and bronchoconstriction, impaired body heat loss and hyperthermia, or upper airway obstruction. Some evidence suggests brainstem abnormalities in areas that mediate ventilatory responses to elevated CO₂, blood pressure, and arousal responses during sleep. However, the relationship between subtle brainstem abnormalities and SIDS remains unknown.

No scientific evidence shows, at least with respect to SIDS, that there is any advantage to the prone sleeping position for healthy infants. Nor does any scientific evidence exist, despite “common sense” fears of aspiration, that the supine sleeping position is harmful to a healthy infant. Since the recommendation of the supine sleeping position for healthy infants, several studies have looked for important adverse effects of the supine sleeping position for infants and found none. Therefore, it is clear from numerous studies that the prone sleeping position is dangerous and that the supine sleeping position is safe for
healthy infants. Evidence also is mounting that the prone sleeping position is particularly dangerous for low-birth-weight and premature infants. For these infants, sleeping position recommendations should be individualized, keeping in mind the risk of asphyxia associated with the prone sleeping position. Similarly, in certain medical conditions, such as gastroesophageal reflux, recommendations concerning sleeping position must be individualized. It should also be noted that the supine position recommendation applies only to sleep. When infants are awake, there appears to be some benefit from spending time in the prone position.

Recent research indicates that infants who are unaccustomed to sleeping in the prone position, who are placed prone to sleep, are a much greater risk of SIDS than infants who are “used to” or accustomed to sleeping prone. The typical scenario is an infant who sleeps on her back at home, but at several months of age is placed prone for sleep at a day-care center or other alternative caregiver setting. Thus, the prone sleeping position is of particularly high-risk for infants unaccustomed to sleeping in this position.

Maternal cigarette smoking is a major risk factor for SIDS. One large study found that 70% of mothers in the SIDS group smoked during pregnancy. A cigarette consumption–SIDS dose–response curve was demonstrated more than 20 years ago. After controlling for other risk factors, maternal smoking at least doubles the risk of SIDS. According to the National Institute of Child Health and Human Development (NICHD) SIDS study, if a mother smokes during pregnancy, the SIDS risk is 3.4 times higher than for controls. Numerous studies have demonstrated that maternal smoking increases the risk of SIDS in a dose-dependent fashion. The more a mother smokes during pregnancy, the greater the risk for SIDS.

In addition, numerous studies demonstrate that environmental tobacco smoke (ETS) exposure increases the risk of SIDS, also in a dose-dependent fashion. Whether expressed as number of smokers in household, number of cigarettes exposed to per day, or hours of smoke exposure per day, study results agree that the risk of SIDS increases many-fold with increasing ETS exposure. When family members smoke in the same room with the baby, the risk of SIDS may be increased 20-fold or more. In countries where the rate of prone sleeping position has been reduced to below 20%, it is estimated that tobacco smoke exposure accounts for approximately 50% to 60% of the remaining SIDS rate. In these countries, where the proportion of infants sleeping prone has been minimized, smoke exposure has become the number-one modifiable risk factor for SIDS.

Pre- and postnatal smoke exposure is particularly dangerous for low-birth-weight and premature infants. Studies demonstrate that smoke exposure dramatically increases SIDS risk in low-birth-weight or premature infants, as much as 60- to 80-fold compared to matched controls not exposed to tobacco smoke. A similar interaction occurs between smoke exposure and the prone sleeping position. Although the prone sleeping position is dangerous for all infants, the risk of SIDS from sleeping prone is magnified when the infant is also smoke-exposed.

Another interesting interaction suggests a role for fetal hypoxia. Maternal anemia greatly enhances the SIDS risk for infants of smoking mothers. An anemic smoking mother appears to be about four times more likely to have an infant die of SIDS than a nonanemic, nonsmoking mother. Such a strong interaction with anemia and smoking during pregnancy suggests that fetal hypoxemia, prenatal nutrition, or other toxicity may play a role in SIDS.

Unsafe sleeping environments, especially soft mattresses or other porous bedding material, increase the risk of rebreathing exhaled gases and therefore the risk of accidental asphyxia and suffocation. The U.S. Consumer Product Safety Commission (CPSC) has reported that as many as 30% of deaths diagnosed as SIDS may have been due to unsafe bedding material. The CPSC has issued specific recommendations concerning crib construction, crib mattress specifications, and bedding materials appropriate for infants. Infants should sleep in an environment that meets CPSC safety guidelines, on a CPSC-approved mattress, without soft or porous bedding material (including stuffed animals, pillows, fluffy bumpers, etc.) that could pose a danger of rebreathing.

Infant–parent co-sleeping may have numerous benefits both for parent and child. However, sleeping in a bed designed for adults may expose the infant to hazardous bedding material such as thick quilts, pillows, or comforters. In addition, infant–parent co-sleeping, when one or both of the parents are smokers, may be associated with increased SIDS risk due to smoke exposure. If parents elect to co-sleep with their infant, every effort should be made to minimize the dangers of having an infant sleeping in a bed designed for adults with bedding material designed for adults.

Several studies have suggested that lack of breastfeeding is a risk factor for SIDS, whereas others have not shown an effect of breastfeeding on SIDS risk. Although breastfeeding clearly has many benefits, its relationship to SIDS remains to be demonstrated.

The role of infection as a risk factor remains in dispute. A variety of bacteria and viruses have been identified from SIDS victims, but no consistent association has been found between SIDS and infection with any particular organism. Parents often report that the infant showed “cold symptoms” just before a SIDS death, giving a general impression that infection is associated with SIDS. However, the NICHD SIDS Cooperative Epidemiological Study analyzed 800 SIDS cases with respect to matched control infants. They found that 29% of SIDS cases had a cold on the day of death, but so did about the same proportion of control infants. Numerous studies have made exhaustive attempts to implicate infection in SIDS, but so far the association between infection and SIDS is unclear.

Serum prealbumin levels, which reflect recent poor nutrition, and the mineral content of bone, an indicator of chronic poor nutrition, have been reported to be normal in SIDS infants. Many studies have tried to identify nutritional deficiencies in SIDS victims; so far, none have been found.

The most potent modifiable risk factors for SIDS are clearly the prone and side sleeping positions and tobacco smoke exposure. In addition, numerous studies have shown an association between SIDS and other maternal and socioeconomic factors such as young maternal age, mother unmarried, less than high school education, low income, inadequate or infrequent prenatal care, crowded living conditions, multigravidity, lack of breast-feeding, and parental drug use. Studies from England have found a strong association between SIDS and such factors as “housing in poor repair,” parents unemployed, poor financial circumstances, and the family not owning their house or not having a telephone. Such associations suggest that, in addition to sleeping position and smoke exposure, other parental or environmental factors are important.

Cocaine exposure may affect neurologic development, possibly leading to postnatal abnormalities of cardiorespiratory control, sleep, and arousal. Several studies to date indicate that the SIDS risk is higher in the infants of substance-abusing mothers; the incidence figures range between nine and 150 deaths per 1,000 live births. Another study, however, found no difference in the SIDS rate between cocaine-exposed and
nonexposed infants. Although drug-exposed infants may exhibit disordered sleep, respiratory control abnormalities, and an increased risk of sudden unexpected death, no study has shown that these factors are causally related to SIDS, and no study has shown that death is related to drug exposure per se.

In the NICHD SIDS study, 43% of SIDS victims had a history of a cold within 2 weeks before death, and 29% of parents reported cold symptoms within 24 hours before death. Sick infants are likely to be treated with over-the-counter medications containing antihistamines, phenothiazines, or other powerful sedatives. One study found that SIDS victims and matched controls had the same incidence of cold symptoms, but infants dying of SIDS were much more likely to have received cold medicine before death. About 25% of SIDS infants had been given phenothiazine-containing cold remedies within 48 hours of death, compared with only 2% of control babies.

Several controlled studies have shown that DPT immunization does not increase the risk of SIDS. However, the possibility of a relationship between SIDS and immunizations is still being actively studied.

African Americans and Native Americans have the highest rates of SIDS in the United States; babies of Asian origin have the lowest. However, no study has been able to sort out an increased SIDS risk due to race per se, as opposed to differences in environmental factors.

Birth weight and prematurity are two of the strongest risk factors for SIDS. The NICHD SIDS study found that infants with birth weights below 2,500 g and below 1,500 g were about 5 and 18 times, respectively, more likely than controls to die of SIDS. Similarly, infants with preterm birth at less than 37 weeks’ postconceptional age and less than 33 weeks’ postconceptional age were 5 and 16 times, respectively, more likely to die of SIDS than controls. Several other studies have confirmed these results. An increased SIDS risk also has been reported in small-for-gestational-age full-term infants.

Despite widespread beliefs about infant apnea and SIDS, to date, no controlled study has shown that apnea of prematurity is a risk factor for SIDS. The NICHD SIDS study found that other neonatal factors such as hypothermia, tachypnea, poor feeding, cyanosis, irritability, fever, and respiratory distress were statistically more likely to have occurred in SIDS infants but were not of predictive value. For decades, the medical literature has erroneously linked prolonged infant apnea strongly with SIDS. However, several studies now have shown that infant apnea does not correlate with subsequent SIDS death.