According to scientific findings, there are two types of smiles infants engage in. The first is the spontaneous smile you will often notice in neonates and the second is the social smile you notice in kids within 6-8 weeks. The baby starts laughing by 16 weeks.
Spontaneous smile shows no kind of emotional content and it occurs in kids from the day they were born to when they are about 4 weeks old. These smiles are quite spontaneous and often occur during the REM sleep stages or when the baby is a bit drowsy.
All baby smiles are said to have a subcortical origin and decreases as the baby's age increases. This means premature babies will smile more than the fully mature babies. Most times, their smile in response to some auditory stimuli and not anything they can see, until they get to about 6-8 weeks. If no external stimulus exists nearby, whenever your baby smiles, it could be what we have always known as the gas smile. If the baby is not up to four weeks old, chances that she might be responding to anything external with the smile are very slim. Evidently, what behind a baby's smile can never be gas. That is nothing more than an old wife's tale.