Newborns smile primarily as a reflex, not because they’re happy or responding to you. These early smiles happen most often during sleep, driven by bursts of brain activity rather than any emotion or social awareness. It’s one of the most common questions new parents have, and the answer has changed a lot as scientists have learned more about infant development.
Reflex Smiles During Sleep
In the first few weeks of life, almost every smile you see on your newborn’s face happens during active sleep (the infant version of REM sleep). During these phases, the brainstem fires off signals that produce a whole range of involuntary movements: rapid eye flutters, grimaces, small weak cries, face and limb twitches, and smiles. These are called spontaneous smiles, and they have nothing to do with social experience or outside stimulation.
The mechanism is straightforward. The brainstem sends out excitatory signals that cause brief muscle movements throughout the body. At the same time, inhibitory signals from the brainstem suppress larger movements so the baby doesn’t thrash around. The interplay between these two systems produces the fleeting twitches and expressions you see on a sleeping newborn’s face. A smile is just one of many involuntary outputs of this process, no different from a finger twitch or a grimace.
Smiling Starts Before Birth
Smile-like facial movements actually begin in the womb. Researchers using 4D ultrasound have observed fetuses producing recognizable smiling expressions as early as 20 to 24 weeks of gestation, alongside other facial movements like yawning, mouthing, and tongue movements. These prenatal expressions appear to be part of the developing nervous system rehearsing the muscle coordination needed for facial communication after birth. They aren’t responses to anything pleasurable; the fetus is simply practicing.
The Shift to Social Smiling
Around eight weeks of age (roughly two months), something genuinely new emerges. Your baby begins producing social smiles: intentional, responsive expressions triggered by something that catches their attention, like your face or your voice. This is a major developmental milestone because it signals that the brain’s higher regions are now mature enough to process social input and produce a purposeful response.
The transition isn’t instant. You may notice your baby starting to smile more when awake, looking at your eyes, or reacting to sounds in the weeks leading up to that two-month mark. Once social smiling begins, it tends to increase in frequency over the following months. Research on visually impaired infants shows that even babies born without sight develop social smiles on a similar timeline, smiling in response to touch, voices, and playful interaction with their parents. This suggests that social smiling is deeply wired into human development rather than something babies learn purely by watching faces.
It’s Not Gas
For decades, parents were told that early newborn smiles were caused by intestinal gas or digestive discomfort. There is no clinical evidence supporting this. The Association for Psychological Science has noted that this old explanation has been thoroughly debunked. Newborn smiles during the first weeks of life are spontaneous brainstem activity during sleep. They aren’t triggered by gas, and they aren’t triggered by contentment. They’re neurological, not gastrointestinal.
Why Smiling Matters for Bonding
Even though a newborn’s earliest smiles aren’t intentional, they still have a powerful effect on caregivers. Infant facial cues, including smiles, activate reward circuits in the parent’s brain that rely on dopamine, the same chemical involved in motivation and pleasure. When mothers view their baby’s smiling face, brain regions tied to reward processing light up. At the same time, interacting with a smiling infant triggers the release of oxytocin, a hormone that reinforces nurturing behavior and strengthens the emotional bond between parent and child.
This creates a feedback loop. The baby smiles (even reflexively at first), the parent feels a rush of warmth and reward, and that feeling motivates the parent to stay close, respond, and engage. Once the baby begins social smiling around two months, the loop becomes genuinely reciprocal: the baby smiles because the parent is there, and the parent responds because the smile feels rewarding. Mothers who produce more oxytocin during these interactions tend to show more consistent, responsive caregiving, which in turn encourages even more social smiling from the infant.
In this way, a newborn’s reflexive smile, meaningless as it is in neurological terms, serves a real biological purpose. It primes the caregiving relationship weeks before the baby is capable of true social interaction, giving parents and infants a head start on the bond that will shape the child’s development for years to come.