A newborn frequently resting on their side is often a source of concern for parents. After nine months curled in the confined space of the womb, many infants naturally gravitate toward a slightly tucked or side-leaning posture. While this preference seems comforting, it raises safety questions for caregivers focused on establishing healthy sleep patterns. Understanding the physiological reasons for this tendency and the established medical recommendations is important.
The Safe Sleep Recommendation
The most important guideline for infant sleep is consistently placing a baby on their back for every sleep period, including naps and nighttime. This recommendation is supported by decades of research and is a core component of the national “Back to Sleep” campaign. Placing an infant on their back significantly reduces the risk of Sudden Infant Death Syndrome (SIDS).
Studies show that infants placed to sleep on their stomach or side have a higher risk of SIDS compared to those placed on their back. Side sleeping is discouraged because it is an unstable position, making it easier for the baby to roll onto their stomach, which poses the highest risk. The highest risk period for SIDS occurs between one and four months of age.
To create a secure sleep environment, the infant must rest on a firm, flat surface, such as a mattress in a safety-approved crib or bassinet, covered only by a fitted sheet. The sleep space must be kept clear of any loose blankets, pillows, bumper pads, or soft toys, as these items present a suffocation hazard. Consistent adherence to the back-to-sleep position is necessary until the baby’s first birthday.
Understanding Why Newborns Favor Side Positions
The preference for a side position in a newborn who cannot yet roll is often due to two primary developmental factors: physiological flexion and the tonic neck reflex. Physiological flexion describes the natural, curled posture that newborns maintain after birth, mirroring their position inside the uterus. This position involves generalized tightness with bent arms and legs, which can cause a baby placed on their back to naturally settle with a slight tilt toward one side.
This temporary tightness provides passive stability for the newborn as they adjust to gravity and begin to develop muscle control. Another contributing factor is the Asymmetric Tonic Neck Reflex (ATNR), often called the fencing posture. When a newborn’s head turns to one side, the arm on that side extends outward, while the opposite arm bends at the elbow.
This reflex creates a side-leaning appearance that mimics a side-sleeping position, though it does not cause the baby to fully roll. The ATNR is one of several primitive reflexes present from birth that usually fade by five to seven months of age. Occasionally, temporary discomfort from gas or mild reflux can also cause a baby to shift their body to find a more comfortable resting posture.
Managing Rolling and Positional Changes
The guidelines for safe sleep change slightly once an infant begins to roll independently, which typically occurs between four and six months of age. The initial instruction to always place the baby on their back for sleep remains mandatory until they reach one year old. This means that even when a baby can roll, they must always be put down initially in the supine, or back, position.
The main adjustment occurs if the baby rolls from their back to their stomach or side on their own while they are sleeping. If they have consistently demonstrated the ability to roll both from back to stomach and stomach to back, they can generally be left in the position they choose. This indicates they have developed the necessary strength and motor control to clear their airway.
Once rolling begins, it is important to stop swaddling. A swaddled baby who rolls over may not be able to use their arms to push up and reposition their head.
The sleep environment must remain clear of all soft objects, loose bedding, and toys, as the risk of suffocation increases once the baby is mobile within the crib. If a baby is placed on their back and is found on their side or stomach, parents do not need to repeatedly reposition a rolling infant who can move freely in both directions. The continued focus should be on maintaining a clear, firm sleep surface.