at what point do I worry about baby's development?

[olivo]

DD2 is 17weeks old and doesnt seem to be doing the things that the other babies are doing (they are mostly a few weeks younger) and looking back at DD1, she doesnt seem to either.she doesn't roll or hold things, she just looks at toys and makes no attempt to grasp things. I know there is a huge a margin for babies abilities to do things, but at what point do i worry about her? (obviously, I am now, but i suppose i mean when should i?!)

I certainly wouldn't worry yet; DS is 7 months developmental age (as was premature) and still isn't rolling, but sits really well now. I think some children don't roll until they are almost ready to be mobile. DS is under a hospital paediatrician and she isn't concerned.
And the toy grasping might take a bit longer too. I would say see GP or health visitor if you are still worried at 6 months.
And avoid the perils of comparing with other children!

oh i know, i must stop comparing but its so hard when other people comment ' oh, i thought she must be younger'
I have to say, i am less worried about the rolling but the toy grasping thing is worrying me.

Oldest son didn't walk till 18months which was the limit I had given myself before I began to officially worried. Did everything late apart from teething.Had "poor muscle tone" As he grew up i discovered he was cautious about everything and had no spirit of adventure at all. He's now 31. He got a first class degree, runs and cycles and is as fit as a flea. Still super cautious and uses green reasons for not going abroad - but i know he really thinks its a bit too risky for him. Happily married with a lovely baby. Try not to worry.

DD is 5 months corrected age (also premature) and has only recently started grabbing toys. She just doesn't seem that bothered really. She loves watching the world around her and that's about it. She has begun rolling front to back, but I reckon that's just cos she HATES tummy time.

She'll hold something for a short while if I put it in her hand, and she'll bat things on her play gym, but won't make an effort to get something. She's just been discharged from hospital physio as her development is bang on where it is supposed to be.

thank you for your reassurances.I guess i am (hopefully) being a bit paranoid, possibly due to the guilt i feel that DD2 doesn't get the attention we were able to give DD1. DD1 is a 'lively' 3yo and takes up a lot of our time. Now i feel vaguely human again, i am going to try and play more with DD2, although, to be fair, she is so different from DD1 and would be happy just to be held all day watching the world go by!