When Babies Start Tummy Time May Matter Less Than Parents Think, Pilot Study Finds

Tummy time has become one of the most widely repeated prescriptions in modern infant care: place your baby on their stomach, awake and supervised, and let them push, lift, and explore. Pediatric guidelines, including those from the American Academy of Pediatrics, encourage supervised prone play beginning early in infancy, with short sessions building to at least 15 to 30 minutes per day by around seven weeks of age. The practice is credited with strengthening the neck, trunk, and upper extremities, promoting head control, and reducing the risk of positional cranial asymmetry. But a quieter question has lingered behind the well-established advice: does it matter exactly when a baby is first placed on their tummy? A new exploratory pilot study, published in Frontiers in Pediatrics, set out to answer that question and found that the answer is far from straightforward.

The research team, led by Anne H. Zachry of the Department of Occupational Therapy at the University of Tennessee Health Sciences, together with colleagues spanning pediatrics, obstetrics, and preventive medicine, followed 58 infants from early infancy through their first birthday. Caregivers reported when they first introduced tummy time, choosing from prespecified categories ranging from the first two postnatal weeks to two months or older, or never. The infants were then divided into three groups: those introduced to tummy time within the first two postnatal weeks (22 infants), those introduced after the first two weeks (21 infants), and those whose caregivers reported never intentionally introducing it (15 infants). Development was assessed at 3, 6, 9, and 12 months using the Ages and Stages Questionnaire, Third Edition, a standardized parent-completed screening tool that measures communication, gross motor, fine motor, problem-solving, and personal-social skills.

The rationale for probing timing is grounded in developmental science. Prone positioning gives infants early opportunities to practice antigravity head and trunk control, upper-extremity weight bearing, postural adjustments, and active exploration of their environment. The early postnatal period is one of rapid neural and motor development, so introducing prone play during this window could, in principle, provide additional practice opportunities and build tolerance for a position many infants initially resist. Prior studies have consistently linked greater cumulative tummy-time exposure with better motor outcomes. One study of 100 caregivers found that four-month-olds with more tummy time achieved certain prone, supine, and sitting milestones earlier. A systematic review of 16 studies involving 4,237 infants across eight countries associated prone play with greater gross motor development, lower body mass index z-scores, and reduced brachycephaly, although the authors noted that most of the underlying evidence was observational and methodologically limited.

What has been missing is evidence on whether the timing of first exposure, independent of the total amount, makes a measurable difference. The new study addressed this gap directly, comparing infants who began within the first two postnatal weeks against those who began later or not at all, and tracking five developmental domains at four ages across the first year. The researchers used Kruskal-Wallis rank-sum tests, a nonparametric approach appropriate for small groups whose distributions may not meet normality assumptions, to compare score distributions across the three groups. They quantified differences with rank-based eta-squared effect sizes, estimated precision with 95 percent bootstrap confidence intervals built from 2,000 stratified resamples, and applied the Benjamini-Hochberg false-discovery-rate correction across the 20 primary domain-by-age comparisons to guard against spurious findings.

The results were largely null. Of the 20 primary comparisons, three produced unadjusted p-values below 0.05: communication at 6 months, communication at 9 months, and personal-social skills at 9 months. After correction for multiple comparisons, none remained statistically significant. The largest observed effect was for communication at 9 months, with an eta-squared of 0.275 and a 95 percent confidence interval spanning 0.100 to 0.507, followed by personal-social skills at 9 months at 0.213 and communication at 6 months at 0.155. These are nominally moderate to large effect sizes, but the confidence intervals were wide, reflecting the substantial uncertainty inherent in a sample of only 32 infants with complete five-domain data at the 3-, 6-, and 9-month assessments, and 30 infants at 12 months. The remaining comparisons produced effect estimates that were small or near zero, with confidence intervals that generally included zero.

One finding in particular caught the researchers’ attention and illustrates the hazards of overinterpreting small samples. At 9 months, the never-introduced group showed the highest mean personal-social score, at 56.9 with a standard deviation of 4.6, compared with 46.3 for the early-introduction group and 46.1 for the later-introduction group. On its face, this seems to run counter to the premise that earlier tummy time confers advantages. But the authors urge caution: the omnibus test does not establish which groups differ, the corrected p-value was not significant, and the never-introduced subgroup contained only eight infants, making its estimates exquisitely sensitive to sampling variability. The researchers also note that the study was designed to examine timing of introduction, not the independent developmental effects of tummy time itself, so comparisons involving the never-introduced group should not be read as evidence for or against the benefits of prone play.

The authors offer a plausible developmental explanation for why a transient signal might appear at 9 months and vanish elsewhere. Around this age, infants typically achieve independent sitting and increasing mobility, which transforms their opportunities for object exploration, caregiver interaction, and environmental engagement. Emerging motor skills have been linked to development in other domains, including communication and language, in prior work. These rapidly changing experiences could influence personal-social scores regardless of when tummy time began. By 12 months, accumulated motor, social, and environmental experiences may have washed out any early differences. Monthly caregiver reports of positioning, restrictive-equipment use, screen exposure, and interactive play showed broadly overlapping patterns across the tummy-time groups, suggesting that infants who started at different ages may have converged in their later experiences.

The study’s limitations are considerable and the authors are unusually candid about them. Attrition was substantial: only 30 of the 58 enrolled infants, or 51.7 percent, had complete five-domain outcomes at 12 months. Baseline comparisons between families who stayed and those who left revealed potentially meaningful imbalances, with standardized mean differences of 0.93 for maternal education, 0.68 for household income and caregiver marital status, 0.57 for paternal education, 0.49 for infant race, and 0.42 for tummy-time introduction group itself. Such differential loss to follow-up raises the risk of attrition bias, meaning the retained sample may not represent the original cohort. Both the exposure, the timing of first tummy time, and the outcome, the ASQ-3, were caregiver-reported, introducing the possibility of recall error and common-method bias. The broad later-introduction category also lumped together infants first exposed at ages ranging from two weeks to two months or older, so the actual temporal contrast between some early and later participants may have been only a few weeks, possibly too small to produce detectable differences on a broad screening instrument.

There is also the matter of what happens after introduction. The study did not standardize subsequent tummy-time practices, so infants who began later may have accumulated similar total exposure by mid-infancy, eroding any differences attributable to timing alone. Infant development is multifactorial, shaped by temperament, tolerance for prone positioning, growth, birth order, parental education, socioeconomic conditions, caregiver-infant interaction, opportunities for unrestricted movement, equipment use, screen exposure, sleep, and environmental stimulation. The sample was too small to support a stable multivariable model adjusting for these factors, meaning residual confounding is likely. The researchers calculated that a properly powered confirmatory study with three similar groups, a two-sided alpha of 0.05, 80 percent power, and a moderate effect size would require approximately 53 participants per group, or about 159 in total, rising to roughly 200 with an anticipated 20 percent attrition.

Importantly, the authors emphasize that current recommendations supporting supervised prone play should not be reconsidered based on these pilot findings. The study was never designed to evaluate whether tummy time itself benefits development, a question already addressed by a substantial prior literature, but rather whether the timing of first introduction matters. The answer, for now, is that there is no clear evidence of reproducible developmental differences by timing, and also no evidence that timing is irrelevant. The findings are best read as a map of uncertainty: preliminary patterns, wide confidence intervals, and a clear agenda for future research. Larger prospective studies with objectively documented exposure, detailed longitudinal measurement of tummy-time dose and quality, and prespecified covariate-adjusted analyses will be needed to determine whether the first fortnight of life truly represents a developmental window for prone play, or whether what matters most is simply the tummy time that follows.

Subject of Research: The relationship between the timing of tummy-time introduction in infancy and developmental outcomes during the first year of life

Article Title: Timing of tummy-time introduction and developmental outcomes during the first year of life: an exploratory pilot study

Article References: Zachry, A. H., Abdullah, F., Assfoura, R., Berke, T., Boushra, M., Ghaly, A., Khalil, M., Woods, L., & Zhao, C. (2026). Timing of tummy-time introduction and developmental outcomes during the first year of life: an exploratory pilot study. Frontiers in Pediatrics, 14, Article 1923077. https://doi.org/10.3389/fped.2026.1923077

Image Credits: AI Generated

DOI: 10.3389/fped.2026.1923077

Keywords: tummy time, infant development, ASQ-3, motor milestones, prone positioning, pilot study, pediatrics, developmental screening, attrition bias, caregiver report, American Academy of Pediatrics, early childhood