S.C.
My son is 8 months old also, and was also born 4 weeks early. He is sitting on his own now, but is not crawling yet. As I know the doctors have told you, since they were born a month early sometimes they are a month behind in certain areas. I have noticed that my son does seem to be behind on some of the "physical" stuff. I didn't think he was ever going to roll over! My doctor has told me not to worry about this, and that by the time he gets to be 12-18 months he should be completely caught up, and you will never know he was born early. Try not to worry, and know that he will catch up eventually! Hope this helps!