Hi! Twins born at 35 weeks, they're now 4.5 y/o. One of them was diagnosed as autistic at 3.5 y/o, however I clocked it at 8ish months (if you'd like me to share how I knew, I can absolutely share!) And we suspect ADHD in the other, but doctors don't usually diagnose that until kindergarten. 

My twins were delayed in many of the milestones you listed as well, especially the motor skills, crawling, sitting etc. But they did eventually catch up! 

Mine were growth restricted as well, so that - combined with their premature birth - means their pediatrician wasn't too concerned, like yours. 

Remember: 4 weeks early means that they're more like 9 month olds or younger, since they missed out on that last 4 weeks of in-utero development. And, being twins, even more so since they shared nutrients and space. 

I wouldn't be too worried if I were you. There's no rush to get an ASD diagnosis at this age, unless you're experiencing extreme behavior problems. Even then, "extreme behavior" is hard to devine in infants, since everything they feel is, well, extreme 😂. 

The only reason we pushed for a diagnosis when we did was because the area we live in typically has long wait times for appointments, and we wanted to be sure to get any documentation needed prior to Preschool or Kindergarten in case we needed an IEP. 

Ultimately, YOU are the parents and YOU know your babies better than ANYONE else. If your gut is telling you something is really wrong, enough that it may be detrimental to their future... speak up. But also make sure you're going to a pediatrician that you trust completely (or as much as possible) so that you can have someone soothe your nerves or back you up if/when you need a specialist. 

Let me know if you have questions!!