Premature twins

I have premature twins of 9 months. They were born prematurely on 7 months of pregnancy. Their birth weights were 1600g and 1900g respectively. And their weights have now increased to 7.4kg and 7.3kg. They had some respiratory problems at the time of birth and spent 4 days in ICU ventilator care.

My problem is that even after 9 months they don't attempt to crawl as many babies do at the age of 7 months. They are active and respond to people, sound, attempt to catch toys, laugh at family, etc. They eat well and utilize breast milk as well. Still they are unable to stand or even crawl on their knees.

How is the development of my premature twins really doing? Would you say they have any developmental problems? What can I do at this stage to help?