Newborns smile mostly during active sleep, the light, dream-like stage where their eyes dart beneath closed lids. These early smiles begin before birth and are driven by bursts of brain activity rather than emotions, though newer research suggests they may not be as random or meaningless as once believed. Understanding what triggers them, and when “real” smiles start, can help you make sense of those fleeting grins in the first weeks of life.
Most Newborn Smiles Happen During Sleep
The majority of smiles in the first few weeks occur during active sleep, the newborn equivalent of REM sleep. In a study using video and polysomnography to monitor 12 healthy newborns over three-hour sleep periods, smiling appeared almost exclusively during active sleep and coincided with clusters of rapid eye movements. Newborns spend roughly half their sleep time in this active state, which is far more than adults do, so there are plenty of opportunities for these fleeting expressions to appear.
The brain’s cortex plays a clear role in generating these sleep smiles. In the same study, researchers observed infants with major brain abnormalities: smiling disappeared only in a baby who was missing nearly all cortical tissue. Infants with partial brain differences still smiled. This tells us that even “reflexive” newborn smiles involve higher brain structures, not just primitive brainstem activity as previously assumed.
They Start Before Birth
Smiling doesn’t begin at delivery. Using 4D ultrasound, researchers have captured fetal facial expressions including smiles as early as 25 to 27 weeks of gestation. The frequency of these expressions changes significantly between the late second trimester and the third trimester, suggesting that smiling develops alongside the fetal nervous system. By the time a baby is born full-term, the neural circuits for smiling have had months of practice.
The “It’s Just Gas” Myth
For decades, parents were told that newborn smiles were caused by gas or digestive discomfort. Scientists have moved well past that explanation. When researchers analyzed newborn facial movements frame by frame using specialized coding systems, they found that smiles appearing as early as one day old often involved not just the mouth but also the cheek and eye muscles. That combination, called a Duchenne smile, is the same pattern seen in genuine adult smiles. This doesn’t necessarily mean a one-day-old is feeling joy the way you do, but it does mean these smiles are more complex than a simple reflex triggered by a gassy stomach.
Newborns also smile in response to specific sensory experiences while awake. Stroking the cheek or belly can produce a smile in the first few days. So can sweet tastes and pleasant smells. These responses suggest some level of sensory processing is involved, even if the baby isn’t consciously deciding to smile.
Awake Smiles May Already Be Social
The traditional view divides infant smiles into two neat categories: meaningless reflexive smiles for the first couple of months, then intentional social smiles starting around eight weeks. Reality appears messier. A study of 40 newborns found a striking difference between smiles during sleep and smiles during interactive waking moments. During active sleep, nearly all smiles were closed-mouth. During interactive waking periods, smiles were overwhelmingly open-mouth, the same style seen in older infants during social exchanges. The ratio was dramatic: 38 open-mouth smiles versus 5 closed-mouth smiles when newborns were awake and being engaged, compared to just 1 open-mouth smile versus 28 closed-mouth smiles during sleep.
Even more interesting, the type of touch the newborns received affected how much they smiled while awake. Babies who experienced continuous tactile contact smiled differently than those who received intermittent or no touch. This suggests newborns may already be responding to social interaction in some rudimentary way, weeks before the textbook “social smile” milestone.
Why Smiling Evolved So Early
From an evolutionary standpoint, a smiling baby is a baby that gets more attention. Attachment theory describes caregiving as an inborn behavioral system in humans: adults are wired to respond to infant cues. Research on how adults react to infant facial expressions has found that people show high positive motivation toward laughing or smiling baby faces regardless of their own attachment style or parenting experience. A crying baby activates a protective, somewhat stressful response. A smiling baby draws people in with almost no psychological cost. Even if a newborn’s smile is neurologically spontaneous rather than intentional, it still functions as a powerful social signal that encourages caregivers to stay close, bond, and provide care.
When Smiles Become Intentional
Social smiling, where your baby smiles in direct response to your face, voice, or interaction, typically emerges between two and three months of age. Around eight weeks, you’ll start noticing smiles that feel different. They happen when your baby is alert and looking at you, not during sleep or at random. They’re responsive: you smile, your baby smiles back. This shift reflects the maturing of visual processing and social brain circuits that allow an infant to recognize faces and connect them to positive feelings.
For premature babies, the timeline adjusts. A study comparing full-term and moderately-late preterm children found that the median age for responsive smiling was 5.5 weeks in full-term babies and 3 weeks in preterm babies when using corrected age (subtracting the weeks of prematurity from the baby’s actual age). Once corrected for gestational age, preterm babies fell within the normal range, so if your baby arrived early, count from their due date rather than their birth date.
When Smiling Patterns Matter
The absence of social smiling becomes noteworthy if it hasn’t appeared by three to four months. Research following infants at higher risk for autism spectrum disorder has found that reduced social smiling can differentiate children who later receive an ASD diagnosis from those who don’t, with differences detectable by 12 months of age. Social smiling on its own isn’t a diagnostic tool, but its presence is one of the earliest and most visible signs that social development is on track. If your baby is smiling during sleep in the early weeks but hasn’t started smiling at faces by around three months, that’s worth mentioning at a well-child visit.