It is so hard to determine 'normal' .... every premie I have had in care while 'tiny' and often riddled with breathing/asthma type issues from under developed lungs has reached the normal developmental milestones on par with other kids because 'development' of that sort begins the moment they are outside the womb and exposed to 'life experiences' that result in social, emotional, cognitive, physical and so forth development.

My own brother was born VERY premature back in the time when this was often a 'death sentence' .... he was jaundice, weighed like 3 pounds, his one lung collapsed during birth and so forth ... however when you look at photos from him back than at 3 months he looked 'normal' just petite but by 9 months of age you would not have known he was a preemie ~ he was the same weight as my cousin who was born 2 weeks before him, he was cruising furniture already and so forth ~ basically once he was out of the womb he quickly 'caught up' because my mother did not treat him any differently than my aunt treated her child.

I have noticed that with SOME preemies or any infant born with 'adversity' in their health .... they are often behind not because of the 'experience' of being born early or with health issues but because their parents 'baby' them way more than children who are born 'full term and healthy' ... meaning that if the benchmark of 'normal' is that you can be sitting unassisted by 6 months than most parents start at about 5 months to engage baby in play activities that support them sitting so they can meet that benchmark but because the preemie is tiny and the parents tend to think of them as well he is only 'really 4 months old' cause he was 2 months premature at 6 months are still 'holding and carrying and protecting the neck' and all the things you do with a 4 month old so therefore the neck and abdominal muscles that WOULD develop just like in a full term infant do not because of lack of 'experience' because their parent is preventing it by their actions and lack of experience for the child ~ hope that makes sense?

Another example of 'normal development' ... I had a child start once who at 3 year olds was still 'crawling' up and 'bumping down on her bum' when using the steps ~ so while this was not normal IMO this was NORMAL to the child cause she had never been allowed any other way as her parent who did not feel confident in allowing her 'walk' up and down kept her in the stage of a 15 month old ... so the kid was amendment she was not 'capable' of doing it any other way initially ~ I had to work past the PARENTS FEAR to get the child to see herself as a competent capable climber as well as the kids using positive peer pressure of see 'X can do it and he is only 1 years old' because I start encouraging them to 'walk' at 16 months because I do not want them knocking each other over bumping down on their bums it is easier to teach them single file, holding the rail, walking one step at a time so once they are competent walkers THIS is how we use the stairs!