Until recently the only sitting up Mary did was in the tripod position. She was like a little bulldog slouched forward and I'd be lying if I said it didn't concern me for a bit.
I do think she was (is) little delayed in her gross motor development, but I think it's just that - a little delay. The pediatrician at her nine month appointment was very reassuring and not worried. He used the words "perfectly healthy" and "just fine" which definitely helped me chill out a little.
And now Mary is sitting up much better. She still reverts to the bulldog or eventually topples over, but each day she has longer and longer stretches of sitting up independently.
And yesterday our little girl started low crawling. It's not quick and she favors one arm more than the other, but she's already getting better and better. Finn is very encouraging which is precious. He will put a toy or something out in front of her and say "Go get it Mary, you can do it. Good girl" (sounds similar to how we praise Roscoe). And honestly I hope she doesn't become proficient too fast - hopefully those chubby little legs of hers will buy me a bit more time before we need to bust out with the baby gates. Probably not though. As she's reminded me - you just can't assume to know what timeline they will be on as it's theirs and theirs alone.