Depends upon what you mean by premature as it relates to autism. 5 1/2 lbs is a large baby in our NICU, so I am going to guess you were born in and around 34+ weeks gestation.
It is true that autism rates amongst the premature average somewhere around 2% overall, and inversely proportional to gestational age (the more premature, the higher the autism rate). Someone born at 22-26 weeks gestation,...1-2lbs,...is far more likely to end up with autism,...I've seen statistics as high as 6% with those babies. It would suggest that the canalicular stage of fetal development (17-26 weeks),...the stage when the vascular system is rapidly developing, that alterations in vascular growth factors (VEGF, etc.) are affecting not only vascular growth, but the subsequent tissue growth to follow. There is data to show that some genetic variants of autism also have alterations in VEGF signaling. In other words, it is less likely that being born "late" preterm will trigger the types of gene transcription that results in the microanatomical changes associated with autism,...as it is later in the brain and vascular developmental process. Rather it is the extremely premature that make up the vast majority of those statistics.
As you probably know, there is no single cause of autism, but rather a mix of some genetic factors and gene transcription that is triggered by things like maternal hormones, inflammatory mediators, etc. In the case of the extremely premature, a few things can happen. A maternal infection, triggers an abnormal rise in inflammatory mediators, that, in turn, can trigger labor and premature birth,...but these same inflammatory mediators can cross the placenta and the blood-brain barrier of the fetus, affecting brain growth. There are many causes of premature labor and birth, but in the end, the baby is now separated from the mother's hormones and cytokine on/off switching responsible for the normal development. In this example, premature infants can be subject to an "injury" effect. The NICU is an obnoxious environment,...the sensory environment is WAY too much for the developing baby,...throw in oxygen levels that far exceed the intrauterine environment,...the potential for infections,...intracranial bleeds,...mechanical ventilators,...IV solutions,...etc. It is a perfect storm for altered brain development.
If we look at autopsy data from autistics, as well as, functional MRI studies, it is quite common to see alterations in neuron formation and migrational patterns in the thalamus, hypothalamus, and cerebellum,...structures that form before the cortex,...so this data suggests the autism effect as early as the first trimester and into the second trimester.
Researchers looking at newly born infants are now able to screen for autism with some pretty simple tests,...albeit they are typically done at hospitals with an associated autism research center, like UCLA, Duke, Johns Hopkins, etc.
So,...what I am suggesting is that your autism was less likely due to your prematurity, per se. By neonatal standards, you would be what we call a "grower-feeder",...totally normal other than perhaps needing some help with your suck-swallow-breathing coordination,...perhaps a week or so of an oral feeding tube. More likely, even if you were born at term, you would have still had autism. In other words, what likely triggered your autism likely occurred several weeks to months before you were born.
PEDS2020032300_proof.pdf (silverchair.com)
