"I have a baby who will be 6 months old in two weeks and I can't help worrying about his development. He was very big and strong from birth (holding his head up and putting weight on his legs within weeks of his birth). We do some tummy time every day but it seems he isn't making much progress towards sitting or crawling. He still completely topples over when he is in a sitting position (I mean completely - he does not sit unsupported even for a couple seconds) and he rarely rolls stomach to back (though he easily rolls back to stomach). Should I be worried? Aren't almost all babies sitting unsupported by 6 months?"
These milestones are a pain in my butt.
Because, yeah, it is helpful to know what's supposed to be happening when in a general sense so you can have realistic expectations (I remember thinking that a one-year-old could probably speak in complete sentences back before I had one of my own), but at the same time kids are so varied and variable that you can really make yourself worry over things that don't mean anything.
Remember, I'm not an expert (sooooo not an expert), but this is what I've gathered from parents of kids with serious problems, not-so-serious problems, and no real problems: The first thing to notice is how your child interacts with you. Is your child alert and mostly happy (unless they're going through a cranky phase)? Will /she make eye contact with you? Does your child smile/laugh? Is your child interested in things? If all these are true, then things are probably going to either be nothing, work themselves out, or be managed in a doable manner. If these things are not true, then please ask your pediatrician for a referral to get your child evaluated, because the sooner you can get information the better off you'll all be.
Now, to specifically address Susan's concerns: The rumor that almost all babies are sitting unsupported by 6 months has been greatly exaggerated. Some are, but a whole bunch aren't. Lots of times kids go from just lying there like pitiful, frustrated little lumps one day to crawling like speed demons and sitting the next. Sometimes they sit first. Neither of mine sat unsupported until several weeks after they started crawling (maybe I'm a bad person, but I thought the slow list to the side and then the topple was consistently funny). Some kids crawl for half a day and then start running. As long as the child is given plenty of tummy time and not confined in a saucer for too much time at a stretch*, they'll develop the way they're supposed to, which may not be on the Official Schedule.
I think that if he's alert and responsive and grabs for things and flaps his arms and legs and all that stuff, he's one of those kids who will crawl at 8 months (yay!) instead of 5 months (quick--babyproof everything in one day!).
Now, as a public service, we will all share how old our kids were when they crawled, sat up unsupported (if you even remember), and walked. Please state gender and if they had any special circumstances. I'll go first: My first boy was born big (9.5 pounds) and crawled at 9 months, sat a few weeks later, and walked at 14.5 months. He is a supergenius. My second boy was born smaller (8.5 pounds), and crawled at 8.5 months, sat shortly after, and walked at 14 months. He is also a supergenius. Both of them run just fine.
(I'm going to go out on a limb and predict that someone will say that her kid crawled at 5 months, and someone else will say that her kid crawled at 11 months, and if we put them next to each other at 3 years you wouldn't be able to tell the difference.)
* I've heard from a couple of sources that the benchmark for saucer time is supposed to be something like 20-30 minutes 2-3 times a day. Which is about as much time as it takes you to shower, and then get a decent start on a meal later on. Or eat an entire dark chocolate bar slowly while reading Entertainment Weekly. If you're saucer-addicted and want to try some options that give more freedom of movement, check out the article on 10 ways to make tummy time more fun from SparkPlugDance.org.