They can be a little behind but not by much. I had my daughter 5 weeks early and she was sitting up by 6 months and crawling by 9 months. She only had them3 or 4 weeks early so they are not that prem. They were also a good weight for twins and early. My little one was only 4lb 9ozMillion percent not defending the fopp but does it make any difference that they were early? Like by corrected age they aren't quite as old as they are by DOB. So mayyyybe they aren't quite at the sitting up unaided stage and flipping over like mad? My son was due almost the same day as them and he's only just started sitting up properly unaided and in the last few weeks he went mental flipping all over the place and backward bum shuffling. Maybe they're just not quite there based on corrected age? I dunno. Like I say, not defending her at all, she really pickles my cheese but god knows it's bloody hard this mumlife so just thought I'd throw it out there?!