The Flip Side of ‘Back to Sleep’

By Phil Stevens MEd, CPO

It's a story that many in our profession have related to on multiple occasions. In 1992, a task force from the American Academy of Pediatrics (AAP) studied the relationship between Sudden Infant Death Syndrome (SIDS) and the prone sleeping position, ultimately concluding "that healthy infants, when being put down for sleep, be positioned on their side or back." 1 Thus was the beginning of the "Back to Sleep" awareness campaign.

In its fundamental purpose it has been largely successful. The incidence of SIDS has been reduced dramatically. However, as many orthotists can attest, this important gain has not been without its lesser comorbidities. The one we tend to think of has been the rapid increase in the incidence of positional plagiocephaly and positional brachycephaly.2-4 However, there have been whispers and rumors of other effects. The purpose of this article is to review some of the other unintended results of the "Back to Sleep" effort.

Initial Reports

Within a year of receiving and implementing the AAP's recommendations, a group of pediatricians from Newton, Kansas, began noticing a change in the developmental screening results observed at four-month well-child checkups. Infants who slept in the supine position appeared to be rolling over at a later age.

To their credit, the physicians took their observations one step further, conducting a retrospective analysis of the roughly 350 infants seen in their office between 1991 and 1995.5 The doctors screened out infants whose weight was low for their gestational age, infants who had been hospitalized for any reason other than normal newborn care, and infants with incomplete medical histories.

In this particular practice, the Denver Developmental Screening Test (DDST) was administered to each child at his/her four-month check-up. The evaluation tool looks for milestones such as rolling over, pulling to sit, grasping a rattle, and reaching for objects. The results of each infant's performance on the DDST were collected. Additionally, the physicians ascertained the preferred sleep position of each infant. Comparing the collected data, they found that infants who slept on their back or their side were less likely to roll over by their four-month check-up (p < 0.001). Other, potentially influencing factors, such as Medicaid status, ethnicity, whether or not the infant was breastfed, maternal parity, maternal age, and maternal marital status, were analyzed and found to have no bearing on the infant's motor performance.

The authors concluded their study by suggesting that the normal age range for achieving certain motor milestones might need to be fixed at new age ranges according to the sleep position of the infant. For example, according to their data, physicians might reassure parents that delayed rolling over is normal at four months for infants who sleep on their back or their side.

Further Confirmation

A short time later, another, much more elaborate study was published. Researchers from the University of Bristol in the United Kingdom conducted a prospective study that ultimately included more than 10,000 infants.6 Using a questionnaire based on the previously mentioned DDST, they attempted to ascertain any correlations between infant development and sleeping positions. Multiple regression analysis was used to adjust for a slew of potentially confounding variables, including the mother's educational level, housing circumstances, number of siblings, ethnicity, birth, and maternal drinking and smoking habits. Accounting for all these factors, the authors found some significant correlations.

At six months of age, children who slept prone had an increased motor score of 0.38 standard deviations and an increased social skills score of 0.11 standard deviations. The exact implication of those numbers is hard to wrap your hands around. In its simplest terms, the motor and social abilities tested higher for the prone sleepers than for the back sleepers, and those changes were "statistically significant," with P values < 0.0001 and 0.01 respectively. In a society where we all want our kids to be above average, the significance of these findings is perhaps debatable. Fortunately, the disadvantages associated with back-sleeping appeared to be transient. The same research group found no significant advantages between the front and back sleepers at their subsequent 18-month evaluations.

Based on their findings, the authors concluded that there was "no convincing reason to change the current advice on sleeping position." While there were some transient developmental advantages associated with prone sleeping, they were insufficient to justify the increased risk for SIDS.

Influence of Tummy Time?

A third study, published soon after the British effort, approached the question from a different angle, but ultimately supported the same conclusion. Three hundred and fifty-one infants were randomly recruited from the Washington DC area. The families of each infant were advised to place their infant on the side or back for sleep according to the AAP recommendation. Parents then kept two separate logs. The first recorded sleep position for the first six months of life. The second was an infant developmental log used to track infant milestones. It consisted of 18 sequential development items, usually occurring at a rate of one to two new motor acquisitions each month.

From this data, the researchers established two cohorts of infants. In the first were included the 57 infants (16 percent) who spent at least 70 percent of the first five months of life sleeping in the prone position. The second included 97 infants (28 percent) who spent the same proportion of time sleeping in the supine position.

From the 18 developmental tasks, the age of acquisition of the eight major milestones was determined for all participants. These then were compared among the two cohorts using statistical methods to control for other possible compounding variables such as infant size, ethnicity etc.

In general, the researchers found that prone sleepers acquired motor milestones at an earlier age than their supine sleeping counterparts. Significant differences were found in the age of attainment of rolling prone to supine, tripod sitting, creeping, crawling, and pulling to stand (P < 0.05). In other words, prone sleepers were quicker to attain all those milestones which required use of the upper extremities and the muscles of the shoulder girdle. In contrast, there were no significant differences in the age of acquisition of the remaining milestones, rolling supine to prone, sitting unsupported, and walking, all of which require less upper-body strength.

As with their British counterparts, the American researchers were clear in their conclusions: "Although supine sleepers attained some motor milestones as much as one month later than the prone sleepers, it is important to emphasize that they still attained these milestones within the accepted time range for normal." So while the back sleepers demonstrated some early motor delay, these delays were not insurmountable.

A second trend also was discovered through the study. In addition to the logs on sleep position, the researchers asked the parents to record the percentage of time that the infants spent in the prone position while awake. Unsurprisingly, infants who slept prone spent twice as much time awake in the prone position than the back sleepers. This raised the question of the influence of "tummy time" on the attainment of the various motor milestones. Though their low numbers failed to reach statistical significance, the authors did observe that those supine sleepers who spent more time in prone playtime achieved several milestones earlier than the other supine sleepers. In other words, tummy time, as recommended in the AAP's first statement on the subject, appears to partially offset the motor delays associated with supine sleeping.

Positioning Practices

This set the stage for one final study, and as the Brits and the Yanks have both weighed in, it's only fitting to hear from Canada. Recruiting only those infants who were routinely placed in supine to sleep, two samples of four-month-olds (n=71) and six-month olds (n=50) respectively, were selected from a community pediatric clinic in Montreal, Quebec.8 As with the previous studies, parents were given questionnaires and diaries to ascertain the infants' sleep position and the times spent by each infant in the various positions of prone, supine, and supported sitting throughout the day.

However, rather than rely on parental reports of milestone attainment, physical and occupational therapists went to the homes of each family to perform standardized tests of both gross and fine motor performance. The tests utilized were developed and normalized at a time when prone sleeping was the standard. Predictably, in both age groups, the back-sleeping infants fared worse than the normative values for the standardized tests.

However, the elements of interest were primarily with respect to the positioning practices of the infants when they were awake. For four-month-old infants, an average of 42 percent of the day was spent awake, with the time spent being held, in supported sitting, or in supine. More than 30 percent of these infants were never placed on their tummy while awake, and 75 percent had less than 20 minutes of tummy time. The numbers were similar for the six-month-old infants, 28 percent of which were never on the stomach and 50 percent of which were on their stomach less than 20 minutes per day.

Using multiple linear regression, the researchers found that a significant amount of the variability observed in motor performance correlated to the time spent awake in the prone position. In fact, awake prone positioning consistently emerged as the most significant predictor of early motor development among these supine sleeping infants. In other words, tummy time appeared to work, and in this study, the infants that had that time scored better on their motor assessments than those that didn't.

So What?

So what's the take-home from all this? First, supine sleeping does seem to be associated with mild motor delay. The association is so strong that the "normal" ages of attainment for certain milestones may ultimately need to be adjusted. However, the impacts of these delays appear to be transient, and back sleepers appear to catch up to their peers. Additionally, tummy time during waking hours seems to offset some of these delays, but many parents have mistakenly interpreted "Back to Sleep" as "Don't ever place your child on his/her stomach." It's not unreasonable to suppose that many of the children we see in helmets fall into this later category, and are therefore likely to be affected by these mild delays. Accordingly, there is probably some value in knowing the flip-side of Back-to-Sleep.


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