Spending less time prone or on their stomach, since the release of the Back to Sleep recommendations to reduce the risk of SIDS, has caused some delays for infants in picking up some milestones. These milestones including sitting up, rolling over, and crawling. Fortunately, by the time they were toddlers, these delays all seem to disappear no matter how your baby sleeps, so it's likely more appropriate to describe these kids as having a 'lag' in their development and not a true delay. If you want to avoid this 'lag,' you might try some tummy time during the day.
Still, most infants sit up without support when they are between five and a half to seven months old.
Sources:
Functional developmental evaluation. Prerequisite to habilitation. Capute AJ - Pediatr Clin North Am - 01-FEB-1973; 20(1): 3-26
Linguistic and auditory milestones during the first two years of life: a language inventory for the practitioner. Capute AJ - Clin Pediatr (Phila) - 01-NOV-1978; 17(11): 847-53
Clinical linguistic and auditory milestone scale: prediction of cognition in infancy. Capute AJ - Dev Med Child Neurol - 01-DEC-1986; 28(6): 762-71
The Denver Developmental Assessment (Denver II)
Association between sleep position and early motor development. Majnemer A - J Pediatr - 01-NOV-2006; 149(5): 623-629
