I would say this.
You can read all the baby books, magazines, websites, and doctors' articles you want. Nothing will ever fully prepare you for what is to come. Each baby is different. All of those publications fail to realize that babies are PEOPLE with PERSONALITIES. If there is something about your baby that you think is odd compared to other babies' behavior, that DOES NOT mean there is something wrong. It could just be a quirk of their character.
That being said, it's still okay to trust your instincts. If you keep getting this gut feeling that there is something not right, don't brush it off as a quirk. Have it seen about. And don't listen to what everyone else may think. Maternal instincts are REAL. No one has them for your baby but YOU. If you repeatedly feel that something is wrong, do something about it, even if five, ten, or five hundred people say everything is fine.
And THAT being said, babies develop at different rates. If your baby is a little behind developmentally in some areas, don't panic. Being a little slow now does not mean they won't be the top of their class come time to graduate high school. All of those books I mentioned earlier will tell you that they should be doing such-and-such activities at this certain age. Just use that information to see APPROXIMATELY when your baby should be doing certain things. It all goes back to trusting your instincts. If you REALLY think something is wrong, have it seen about.
Good luck to all new Mommy's and Daddy's out there! I have to say from very recent personal experience...it's quite a ride, and it's well worth it!