It is truly an amazing feeling; one I have never experienced with either of my other children. I think it has to do with overcoming odds, and knowing how hard my little girl has to work to achieve each milestone. As a result of Charlotte's bonus chromosome she has low tone; meaning she has to work extra hard to strengthen her muscles in order to hold her head up, sit, crawl, walk, and every other skill that comes easily to many of her typical peers.
Luckily, therapy around our house is just glorified play time. We put on Charlotte's little pink hot pants (a.k.a. "hip helpers") and hang out on the floor for awhile. Thankfully, I have lots of help.
Yes, if only I had know about those feelings, maybe the first few days after Charlotte's birth wouldn't have been quite so hard. Hindsight is 20/20.