EDIT: We also determined that certain factors may affect it.
One interesting point we haven't discussed here yet is that children are evaluated from birth in "scoring points" regarding their development. In evaluating the evidence, we have to keep in mind that there is a "norm" we are not told. These are all skills the babies are supposed to develop themselves. Some babies do much better than the "norm". Some do much worse. Some may be much better in one "norm", while they are worse in another. Here is a list of the most severe "red flags" (rather limited in its value for the future of the baby) for different stages in the development. If these don't happen within the expected time frame, you should contact a specialist. No guarantees that this will be solved eventually. Not knowing or not noticing any of these at the right time may have serious consequences for how the baby is going to develop further. We should also keep in mind that these may appear in all layers of society all over the world, regardless of race, sex, environment, opportunities, etc.:
1 month:Feeds slowly or doesn't suck well
Doesn't seem to focus her eyes or watch things moving nearby
Doesn't react to bright lights
Seems especially stiff or floppy
Doesn't respond to loud sounds
3 months:Can't support his head well
Can't grasp objects
Can't focus on moving objects
Doesn't smile
Doesn't react to loud sounds
Ignores new faces
Seems upset by unfamiliar people or surroundings
4 to 7 months:Seems very stiff or floppy
Can't hold her head steady
Can't sit on her own
Doesn't respond to noises or smiles
Isn't affectionate with those closest to her
Doesn't reach for objects
8 to 12 months:Doesn't crawl
Seems to drag one side while he's crawling for a month or more
Can't stand with support
Doesn't try to find objects you've hidden in front of him
Doesn't say any words
Doesn't use gestures, such as shaking his head "no" and pointing
13 to 24 months:Can't walk by 18 months
Doesn't understand the use of everyday objects
Doesn't speak at least six words by 18 months or two-word sentences by 24 months
Doesn't imitate words and actions
Doesn't follow simple instructions
Loses skills he previously had
25 to 36 months:Struggles with separation anxiety
Doesn't interact with people outside her family
Doesn't play with other children
Avoids eye contact
Can't throw a ball or jump
Can't climb stairs with alternating feet
Has trouble scribbling
Doesn't use more than three words in a sentence
Isn't able to complete a sentence
Is often difficult for strangers to understand when she talks
Doesn't play make-believe
Excessively balks at basic self-care, like getting dressed or going to sleep
Loses skills she previously had
Source:
https://www.babycenter.com/milestone-charts-birth-to-age-3