This study aimed to determine whether supine sleep-positioned infants have delayed motor skills at age 4 and 6 months, and if delays are associated with decreased exposure to prone position. One 4 and one 6-month-old sample of healthy infants born at term were recruited. Motor assessments included the Peabody Developmental Motor Scale (PDMS) and Alberta Infant Motor Scale (AIMS). Parents completed an infant behavior diary for 3 consecutive days. Seventy-one 4-month-old infants were assessed (38 males; mean age 4.4mo, standard deviation [SD] 0.2). Mean motor scores were close to normative standards (AIMS 47.7, SD19.6; PDMS gross motor quotient [GMQ] 96.3, SD 6.5; PDMS fine motor quotient [FMQ] 99.2, SD4.8). No infant scored below cut-off values used to identify motor delay. Milestones less likely to be achieved included extended arm support in prone, hands to feet in supine, and sitting with arm support. Exposure to ‘tummy time’ while awake was correlated with AIMS scores (r=0.38, p<0.01). Fifty 6-month-old infants were assessed (21 males; mean age 6.4mo, SD0.4). Mean scores were shifted down for all scales, and as much as 1SD for PDMS (AIMS 44.5, SD 21.6; PDMS GMQ 85.7, SD7.6; PDMS FMQ 88.9, SD9.0). Only 22% of 6-month-olds could sit without arm support versus 50% expected in a normative sample. Remarkably, 22% of our sample exhibited gross motor delays (quotient<78). Tummy time (awake) was significantly associated with the AIMS (r=0.64) and PDMS GMQ (r=0.55) and FMQ (r=0.33) quotients, even after adjusting for confounders. Typically developing infants who were sleep-positioned in supine had delayed motor development by age 6 months, and this was significantly associated with limited exposure to awake prone positioning. This has important implications for interpreting motor assessments of infants at risk and for preventing inappropriate referrals.