Premie moms

Hi preemie mum Here… you follow their corrected age for their development

Mine was 10 weeks early

If before 34 weeks, you use whatever their age would be from 34 weeks (or whatever week they were born).
Sitting on her own is a 9 month milestone so there’s no reason to be stressed in general. Sitting happens in three stages tripod, wobbly and unassisted.
In terms of rolling while it’s considered a 6 month skill there usually isn’t really a reason to be concerned before 9 months (unless you know something is going on)
It wouldn’t hurt to go to a pedantic physio but 🤷🏻‍♀️ Sounds like she’s within her milestone range

mine was 6 weeks early. You base off corrected age. But also sometimes they may need more time. Mine didn't get her first tooth untill recently and she's about 1 year and 6 months. She still isn't standing on her own or walking on her own, but she is trying in the house all the time. She will walk if I hold her hands no problem. She also is only 17 pounds but she was born 4 pounds. So don't forget your doing your best. I was having a hard time accepting it. I know you ask all that but I went throught this and I wanna pass on the encouragement I got. As long as they are healthy and happy try not to stress. But the answer to your question is the corrected full term 40 week is when you start counting for that stuff. The early weeks before the 40 week mark isn't really counted as far as that aspect.