Children develop at different rates, however for the most part they will fall under a general time line. If your baby is premature, the time line they fall into will be in accordance with their gestational age, not their birth age, which means they may be behind by a few weeks or months. If your child seems to be behind, or you are worried about their development it is wise to talk to your pediatrician. There are certain milestones that your child should meet within a few weeks of the average. If they are not meeting those milestones, they might be experiencing developmental delays. The sooner you catch developmental delays, the better chance your baby has of treatment being effective.
The first thing your pediatrician will tell you is to trust your gut or instincts on the matter. If something seems odd or wrong to you, pay attention to that feeling, you know your baby better than anyone else. If you have any concerns at all, pick up the phone and ask your pediatrician about it. If nothing else, you get the peace of mind of knowing you did not let something slide that should have been looked into.