Hi Albaba,
I read your message and though my DCDA boys are only 9 months, it could have been me writing the very same thing 6 wks ago. My eldest has been right on(if not ahead)of all his gross motor milestones but my little one is about 3 months behind and seemed to be lagging even further behind all the time. Luckily I put the boys on my BUPA as soon as they were born so we were seen very quickly after our Health Visitor and GP raised concerns. The Paed we saw also said low muscle tone in trunk and legs. We had numerous blood tests (for a muscle enzyme issue - do not google, I scared myself half to death) and were also referred for an MRi.
We started physio and within 4 days our little one started to roll from back to front, which he'd never been able to do before. Within 2 weeks he was starting to sit unaided. A massive improvement in a very short amount of time. The MRI wasn't nearly as bad as we feared and 2 weeks later he was given the all clear. The muscle tone in his trunk and legs has improved vastly and though he still does have head lag, he's improving all the time.
Please don't worry. I was terrified about CP too but if your little one is passing toys from one hand to the other and uses both sides of her body fairly equally, I'd say this was unlikely. I also think you might have noticed that by now.
Our little one had a fairly traumatic birth, having slipped into transverse they had to cut through his placenta to get him out which caused me to lose a hell of a lot of blood and for him to have quite a shock. They were concerned that this may have caused some damage to his brain but it turns out all is fine and he's just a lazy bones!
I really hope you have a positive outcome. Try not to worry too much. If you are already seeing improvements as a result of the physio, then the chances are your little girl is just a little bit too chilled out and needs a little helping hand to get her to catch up with her sister. We will never know why our little one is so behind his brother but they are confident that he will catch up eventually. Best of luck.
Plum