INTRODUCTION AND PURPOSE
Independent sitting is one of the earliest developmental markers of postural control.1 Between 5 and 7 months of age, the typical infant attains adequate head control, trunk strength, and neuromuscular coordination to maintain sitting with hands free.1 Attainment of this milestone creates a developmental cascade by altering what an infant can see, attend to, and do. The developmental cascade theory hypothesizes that advances in motor skills drive and predict changes in cognitive, social, perceptual, and language domains.2,3 Libertus and Violi,2 in a longitudinal study of 29 infants developing typically from 3 to 14 months, demonstrated that sitting was a significant predictor of language abilities at 10 and 14 months of age. Similarly, in children at high risk for autism spectrum disorder, LeBarton and Iverson4 found a significant correlation between sitting at 7 months and 2 important social-communication milestones: duplicative babble and show gesturing. The latter is a behavior in which an infant shows an object to a social partner inviting commentary.4
Visual attention and object manipulation are essential for cognitive development.5,6 Both are linked to the onset of independent sitting.5,6 Harbourne et al5,6 analyzed duration of visual attention, or look times, in 6 months olds developing typically at different stages of sitting ability. Look times are known correlates of cognitive performance as well as later intelligence and represent an infant's ability to extract and process information from a visual stimulus.5,6 Shorter look times reflect more efficient information processing and are associated with higher cognition.5,6 Harbourne' s team6 reported that infants who maintained sitting with hands free for greater than 5 minutes demonstrated significantly shorter look times than same-age peers who were unstable in sitting. Their conclusion was that the intake and processing of visual information was related to and accelerated by postural control in sitting.6 Similarly, infants capable of hands-free sitting reach for and manipulate objects with greater frequency and ease.1,7 Enhanced visual, sensorimotor, and tactile exploration while sitting upright builds cognitive and perceptual understanding of 3-dimensional form, object function, figure ground, and temporal-spatial relationships.2,8,9 An iterative cycle of perception, action, and learning simultaneously advances multiple developmental domains.3,5,10
Verticality further alters an infant's social perspective.2,4 Independent sitters experience increased opportunities for joint attention and caregiver interaction.2 Iverson and colleagues4,11 demonstrated that sitting ability in 5- to 7-month-old infants at risk for autism spectrum disorder was correlated with the onset of gestural communication. Gesturing, paired with vocalization, is a social skill that infants use to gain and direct adult attention.4 Independent sitting increases an infant's gestural communication, object sharing, social referencing, and turn taking: all related to both language and social domains.2,4,11 In a review of developmental linkages between motor and language domains, Iverson11 argued that motor experiences such as sitting organize general communication skills and language, specifically by changing to what an infant attends and what he or she learns contextually from social partners.
Compared with infants born full-term, infants born very to extremely preterm (before 32 weeks' gestation) demonstrate greater risk for delays in multiple developmental domains, including motor.12–15 In a cross-sectional sample of 308 infants born prematurely, Formiga and Linhares16 identified consistent motor delays in the first year. These authors suggested that (1) motor norms for infants born full-term were not applicable to infants born prematurely who demonstrated a different and distinct neuromaturational trajectory; and (2) motor delays were expected with infants born preterm as a result of prematurity-related health risks.16 Yet, if motor skills drive developmental cascades, even subtle delays, such as later sitting onset, may compound deficits in other domains by constraining sensorimotor, perceptual, communication, and social opportunities. These constraints, in turn, may amplify developmental sequelae for infants born early and at-risk medically whose early learning and exploratory abilities are known to be aberrant.17–19
The purpose of this retrospective study was to examine differences in prematurity-related risks and compare cognitive and language outcomes between sitters and nonsitters at 6 months' adjusted age in a cohort of infants born before 32 weeks' gestation. Based upon evidence for developmental cascades associated with independent sitting in infants developing typically, it was hypothesized that prematurity-related risk profiles (gestational age at birth, birth weight, multiple birth, gender, race/ethnicity, and number of days with respiratory support), and Bayley Scales of Infant Development-3rd edition (BSID-III)20 cognitive and language composite scores, would differ between sitters and nonsitters. In addition, it was hypothesized that independent sitting at 6 months' adjusted age would be associated with higher BSID-III language and cognitive composite scores.
The Institutional Review Board granted approval for this retrospective study. Data were collected from an electronic database of infants whose families voluntarily enrolled in a state-funded, neonatal intensive care unit (NICU) developmental follow-up program. Infants were graduates of 4 geographically distinct, tertiary care NICUs between 2012 and 2016. A comprehensive developmental assessment was provided at 6 months' adjusted age if criteria for moderate or level II-defined risk were documented. The level II risk factors include but are not limited to the following: birth weight of less than 1500 g, birth before 32 weeks' gestation, sepsis, therapeutic hypothermia, or respiratory distress requiring assisted ventilation.21 Infants classified in level II, though at-risk for developmental sequelae, do not qualify automatically for early intervention services.21
The cross-sectional cohort (n = 105) chosen for analysis met the following inclusion criteria: birth on or before 320/7 weeks, developmental follow-up assessment between 5 months 16 days and 6 months 15 days' adjusted age (a distinct BSID-III developmental window),20 complete data sets containing original BSID-III and motor screening protocols, and no record of ongoing early intervention or physical therapy. Figure 1 provides information on the selection of the study sample from the full cohort of available electronic records. Infant demographic and medical risk profiles as well as 6 months' BSID-III cognitive and language composite scores were retrieved. The BSID-III is a validated norm and criterion-referenced test of development used to determine delays or need for services.20 The categorical demographic variables of interest included gender, race/ethnicity, and type of insurance (private or exclusively Medicaid) as an indicator of socioeconomic status. Multiple or singleton birth and head ultrasound (HUS) results were the medical risk variables. The HUS results were dichotomously classified as either normal or abnormal, with the latter reflecting a documented history of a grade intraventricular hemorrhage, periventricular leukomalacia, hydrocephalus, or other brain anomaly. The continuous variables of interest were gestational age at birth, birth weight, number of days of respiratory support, weight upon follow-up, and BSID-III cognitive and language composite scores. All of these variables were selected for known correlations to prematurity-related medical complications such as bronchopulmonary dysplasia, retinopathy of prematurity, cerebral palsy, or developmental risk.22,23 Infants were classified as sitters if they passed the BSID-III Motor Screening item (item #11) described as: “the ability to sit for more than 60 seconds while playing with hands free.”20 This criterion-referenced motor item represents the highest level of sitting skill captured by the BSID-III motor screener in the 6 months' developmental window. Although its duration is shorter than Harbourne's definition of independent sitting (5 minutes with hands free),6 maintaining sitting for 60 seconds captures the essential elements of independent sitting: upright posture, hands free, and stable balance.
Statistical analyses were conducted using SPSS version 24. Descriptive statistics were used to summarize continuous and categorical variables of interest. Independent t tests with Bonferroni correction or analyses of variance were used to compare continuous variables as appropriate and χ2 analysis assessed differences in categorical variables between groups (sitters vs nonsitters; inclusion sample vs cohort lost to follow-up). Group differences in BSID-III language and cognitive composite scores were assessed according to demographic and prematurity-related risk variables. Pearson's r, univariate and stepwise regression analyses were used to determine whether significant correlations (r > 0.2, P < .05) existed between BSID-III composite scores and any demographic and prematurity-related risk variables.
Figure 1 depicts the final study sample (n = 105) based upon eligible electronic records and the full inclusion criteria. A descriptive summary of infant characteristics by cohort (attended clinic/met inclusion criteria vs lost to follow-up) is in Table 1. Mean differences between groups indicate that infants lost to follow-up were 99 g lighter at birth (standard error [SE] = 39.65, P < .05) and spent 4 days less with respiratory support (SE = 1.75, P < .05). Infants of black race/ethnicity and with Medicaid only as insurance payment were significantly less likely to return for follow-up (χ2 = 26.14, P < .001; χ2 = 12.501, P < .001, respectively).
TABLE 1 -
Comparison of Prematurity
-Related Risk Factors and Demographics Based on NICU Follow-up
||Attended Clinic (n = 105)
||No Show or Lost to Follow-up (n = 401)
||Mean (SD), SE
||Mean (SD), SE
||1359 (401), 39
||1260 (351), 18
||29.01 (2.24), 0.218
||28.72 (2.03), 0.101
|Days on ventilationa
||14.7 (18.20), 1.78
||10.63 (15.37), 0.77
Abbreviations: BW, birth weight; GA, gestational age; HUS, head ultrasound; NICU, neonatal intensive care unit; SD, standard deviation; SE, standard error.
aSignificant differences using an independent t test with Bonferroni correction, P < .05 level.
bSignificant differences using a χ2, P < .05 level.
Approximately one-third (31.4%) of infants in the inclusion sample were classified as sitters at 6 months' developmental follow-up. The proportion of sitters to nonsitters did not differ significantly by gender (χ2 = 2.053; P = .152), multiple birth (χ2 = 1.752; P = .416), HUS results (χ2 = 1.513; P = .219), payment type (χ2 = 2.803; P = .094), or race/ethnicity (χ2 = 1.380; P = .502) (Table 2). Mean differences indicated that nonsitters were born earlier (−1.18 weeks, SE = 0.458, P < .01), weighed less at birth (−183 g, SE = 82.7, P < .05), and spent more days on the ventilator (+7.52, SE = 0.377, P < .05). The BSID-III cognitive and language composite scores were respectively 6.3 (SE = 1.89) and 7.7 (SE = 2.05) points higher in sitters (both significant at P < .001) (Figures 2 and 3). The BSID-III cognitive and language scores did not differ significantly (P < .05) according to gender, multiple birth, payment type, race/ethnicity, or HUS results.
TABLE 2 -
Comparison of Infant Characteristics According to Sitting Ability
||Did Not Sit Independently
||(n = 33)
||(n = 72)
||Mean (SD), SE
||Mean (SD), SE
||1484.52 (343.56), 59.8
||1301.68 (413.88), 48.77
||29.8 (1.83), 0.318
||28.6 (2.32), 0.274
|Days on ventilationa
||9.55 (13.39), 2.33
||17.07 (19.65), 2.32
|Weight at follow-up, g
||7765.8 (907.91), 158.05
||7426 (912.36), 107.52
Abbreviations: BW, birth weight; GA, gestational age; HUS, head ultrasound; SD, standard deviation; SE, standard error.
aSignificant differences between sitters and nonsitters using an independent t test with Bonferroni correction, P < .05 level. No significant differences were found between sitters and nonsitters within any of the above categorical variables using χ2 analysis, P < .05 level.
Point-biserial correlational analyses indicated that sitting was inversely related to chronological age (r = −0.198, P < .05) and days on assisted ventilation (r = −0.193, P < .05). However, it was directly related to cognitive and language composite scores (r = 0.313, P < .01; and r = 0.348, P < .01, respectively). Sitting was the only variable significantly associated with BSID-III cognitive scores (R2 = 0.117, F (1,138) = 18.28, P < .01). Children who were sitting at 6 months of age were more likely to have higher BSID-III cognitive scores, with sitting accounting for 11.8% of the variance in cognitive domain scores (β = 0.342, P < .01). Days on ventilation was the single most significant variable associated with BSID-III language scores (R2 = 0.122, F (1,138) = 19.16, P < .01). Fewer days of ventilation resulted in higher language scores (β = −.349, P < .01). However, with the addition of sitting, the variance accounted for in BSID-III language scores increased (R2 = 0.168, F (2,137) = 13.86, P < .01). Fewer days on ventilation (β = 0.287, P < .01) combined with sitting (β = 0.224, P < .01) accounted for 16.8% of the total variance in BSID-III language scores.
Infants born prematurely are at higher risk for global developmental delays even in the absence of cerebral injury or major medical complications.24 Cognitive, perceptual, and motor challenges in the first 6 months are well substantiated in this population.17,18 In a comparison of 20 infants born before 28 weeks' gestation and 20 infants born full-term, Zuccarini et al25 reported that infants born prematurely examined at 6 months' adjusted age demonstrated “less time in manual engagement, active manipulation, mouthing, and turning/rotating” of objects during a 5-minute, mother-infant play interaction than their full-term peers. These same infants also scored lower on tests of general psychomotor development, establishing that a close relationship exists between early psychomotor abilities and manual dexterity.25 Similarly, Lobo et al18 reported decreased and less variable exploratory behaviors in the first 6 months, and decreased visual-haptic multimodal exploration in a cohort of infants born preterm and observed longitudinally from term-adjusted age to 2 years of age. The existing evidence suggests a strong link between motor abilities and other developmental domains in infants born preterm as young as 6 months of age.18,25
In the current study, BSID-III cognitive and language composite scores were statistically higher in at-risk infants born preterm who sat independently at 6 months' adjusted age, further implicating an early developmental cascade associated with upright postural control. As substantiated by Harbourne et al,1 Ryalls et al,26 and Iverson et al,4,11 advances in manual and visual exploration are linked to the onset of independent sitting regardless of an infant's age. Thus, perceptual-motor experience, and not simply neuromaturation, influences cognitive and language development.2,3,11 In a neuromaturational model of development, motor, cognitive, and language advances are considered relatively unrelated, even competitive developmental processes attributed to maturation of the central nervous system. As Iverson11 aptly states, this view has led to a “long-standing, widespread but empirically unverified belief among parents, pediatricians and even some developmentalists that when children are in the process of acquiring new motor skills, progress in language (or other developmental domains) comes to a halt.” However, the reverse is more likely. Consistent with the concepts of embodiment and grounded cognition, changes in postural control and motor abilities actually drive changes in cognitive and language domains by scaffolding perceptual skills and problem-solving opportunities.3
The larger question for our findings is whether or not a 6- to 7-point difference in BSID-III scores between the groups is clinically significant. Given that cognitive and language composite scores fell within the normal distribution range (100 ± 15 points) for both groups of infants, should statistical significance be disregarded? According to Anderson and Burnett,27 the BSID-III, though widely used for its strong psychometric properties, overestimates developmental abilities and underidentifies delays. In a cohort of infants developing typically from Australia, cognitive and language scores were 8 to 18 points higher than the reported BSID-III norms for a given age.27 This suggests that infants in the current study may have developmental delays compared with their peers developing typically, and nonsitters would have scored nearly 1 standard deviation below the normative range if the recommendation for a 7-point norm inflation were considered.27 The second concern is that developmental gaps in language and cognitive abilities of infants born prematurely have been shown to increase with age.27,28 With the potential for underestimation of abilities and a widening developmental gap over time, subtle delays at 6 months of age should not be ignored.
In this study's cohort, the ratio of sitting to nonsitting infants remained consistent across gender, multiple birth, race/ethnicity, and insurance payment-type groups. Similarly, no significant differences in BSID-III scores were found between these respective groups. Previous findings consistently support that variance in cognitive and language skills in early development was more closely tied to family demographic variables associated with socioeconomic status, or to cultural practices related to race/ethnicity than to any other variable.15,29 Our findings contradict this, suggesting instead that postural control in sitting plays a critical role in advancing cognitive and language abilities in moderate-risk infants born preterm regardless of their demographics or race/ethnicity. However, a disproportionate number of infants who were black and those whose families had Medicaid-only insurance failed to return for NICU follow-up and this most likely skewed the demographic distribution of the sample.
Medical complications, which impact movement abilities, are a known factor contributing to developmental outcomes in infants born prematurely.17,18 Indeed, our findings suggest that sitters differ from nonsitters with regard to medical risks such as gestational age at birth, birth weight, and length of assisted ventilation. Furthermore, these same factors, all tightly interrelated, are associated with greater risk for adverse cognitive and language outcomes.13 If the sitting ability at 6 months' adjusted age is predictive of emerging cognitive and language delays as suggested by the work of Libertus and Violi2 and Iverson,11 the critical window for early identification of and intervention for movement difficulties lies between NICU discharge and 6 months' adjusted age follow-up. Best practice recommendations for early identification of motor disorders currently advocate for follow-up at or before 3 months of age for any infant identified as at risk for movement delays to maximize their potential to capitalize on plasticity through intervention.30
While our findings support the concept of developmental cascades related to sitting, several limitations should be acknowledged. A retrospective, cross-sectional design does not establish causal relationships. While 6- to 7-point difference in BSID-III cognitive and language composite scores at 6 months' adjusted age is considered statistically relevant, extrapolating how this relates to later developmental outcomes is tenuous, given the nature of developmental variability in a population of infants born preterm. Our data lacked direct measures of socioeconomic status, and the no-show rates were higher within demographic groups that may be of low socioeconomic status. The generalizability of the results is, therefore, limited. Future work needs to consider prospective, longitudinal studies to determine whether early sitting is associated with later cognitive and/or language skills in infants born preterm, or if other mitigating factors associated with motor experience such as the amount of playtime on the floor, general activity level, or others mediate the developmental outcomes.
Sitting ability at 6 months' adjusted age is associated with cognitive and language abilities. For infants born preterm and at a higher risk for developmental delays, early identification of and intervention for motor difficulties, even if subtle, may be important to prevent broader delays across developmental domains. Family education programs emphasizing strategies for motor exploration, postural control in sitting, and perception-action opportunities from birth are important for promoting broader development in moderate-risk infants born before 32 weeks' gestational age.
Special thanks to the Tracking Infant Progress Statewide (TIPS) team who, with unparalleled foresight, built a comprehensive database for monitoring outcomes in at-risk, premature infants; thank you to all the families who voluntarily participate in this NICU follow-up program to ensure the developmental well-being of their infants.
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