Unborn babies can start smiling as early as 13 to 14 weeks into a pregnancy. In fact, some studies suggest that fetuses may even be able to respond to flavors in the womb, with fetuses exposed to carrots more likely to smile and those exposed to kale more likely to cry.
Campbell (2004) shows a smile of a fetus at 33 weeks of conception. Piontelli (2010) mentions that fetuses start to smile from 13 to 14 weeks of conception, and the smiles increase from 16 to 18 weeks.
Source (sciencedirect.com)
Research has shown that fetuses can make reflexive facial expressions, including smiling, as early as 24 weeks of gestation, observed through 4D ultrasounds. However, these expressions are not socially driven but rather reflexive responses to stimuli. For example, a study by Durham University found that fetuses exposed to different flavors, such as carrot or kale, displayed laughter-like or crying-like facial expressions. These reactions indicate reflexive neural activity, not emotional or social smiling.
Which is what I said above.
Ehhhhhhhhh kind of. I just wanted to draw the distinction, which probably wasn't clear in my original comment on the video, that there are two categories of smiles - reflex or social. I wanted to clarify that the baby was not smiling in a social manner like you or I would, rather it was a reflex.