Any new mom is filled with fear that SOMETHING is going to be wrong with her child. Anything. We spend hours wondering what milestones are appropriate at each age, observing what other children are doing and stressing out that our children are deficient in some way. If you have had a few children, your friends who are new moms will do this, and they will seek your advice for all of these fears. Now I want to give some advice to you: shut it. Unless your advice is: (1) my kid was even later in doing X, and is totally fine; or (2) that is TOTALLY normal and every kid does it, your advice blows. Keep it to yourself. Even if this kid has two heads, and you are pretty sure most kids don't have two heads at six months, don't do it. Assume she has a pediatrician, and that it isn't you... unless, of course, it is you, and then you can go for it.
When my first child was about two, I started to have some suspicions that something about him was just weird. He was really obsessed with particular things, chanted a lot to himself, sat awake at night making repetitive letter sounds, and walked around with a writing pad all the time, which he would use to write and erase the same stuff over and over. It was, admittedly, unusual. So I mentioned it to a couple of other moms. "Hey, this is really weird, right? I think he might have OCD or something." I did not go to his pediatrician, because this did not conveniently pop up right before a well-baby visit and would require taking time off of work, so I did what any good mom does. I talked to my friends and I consulted the internet. The friends who did the right thing told me "it's totally normal." I appreciate it, girls... and I know you were probably like "what the hell..." once I was gone. Others told me it wasn't. Yes, I suspected OCD. Then one mom told me she thought he was autistic. She has no medical degree, but to her he showed a lot of signs of it.
Well, in a state of panic, I... right, googled. Sure enough, some of this introverted repetitiveness can be a sign of autism or things on the autism spectrum thingy. I panicked. Here I have been sitting ridiculing moms who were not vaccinating their kids because Jenny McCarthy thought it was a bad idea, and this is karma seeking revenge on my pompous ass. Eventually, I made an appointment with the pediatrician. He outright laughed at me. He thought the question was absurd, as my precocious toddler showed no signs of autism, or OCD, for that matter. You know what? Two year olds are learning rapidly and trying to retain information. Particularly bright toddlers like to repeat the stuff they are learning to themselves, especially during quiet times, like alone in their rooms.
I am sure this mom meant no harm, and none of us do. But moms do it all the time, myself included. You have a few kids and you think you are some sort of expert on childrearing and child development. Leave it to the experts. I try to catch myself, remembering that experience, and 1) try not to panic when I see anything somewhat different in one of my kids; and 2) avoid doing the same thing to other moms. If your mom friend expresses concern that her toddler does not walk, and your present olympian was older and did not walk, share. She will be thrilled to hear it. If you never saw a kid that age who did not walk and you are sure he has polio, keep it in the vault. Tell her to talk to the doctor if she is concerned, but young Johnny seems totally normal to you. If young Johnny turns out to have some issues, support her. Until then, keep your unconstructive diagnoses to yourself.