4.5 month old, arms and head shake when reaching for toys...

NewMrsMacC

I am very concerned, I have called our pedi and he says this can be due to an immature nervous system, and if I continue to be concerned to get it on video and bring my daughter in. Let me describe what she is doing, and please tell me someone else knows  something about what I am seeing...

When my daughter is laying on her back (she is 18 weeks old) and she reaches out (up) for a toy with both hands.. As in, if I hold the toy above her and she stretches her arms out to her midline and reaches for the toy, her arms shake- and sometimes her head does as well. Her eyes do not flicker of flinch or do anything strange..But as she is concentrating on the toy, trying to reach for it above her with both hands/arms her arms shake. It is not a full or hard shake, its a mild tremble, but noticeable. It doesn't last for long (only a few seconds) but she does do it almost every time she lays on her back and reaches out in front of her for a toy being held above her. 

She doesn't do it when sitting up and reaching out, just when on her back and reaching up.

Please tell me someone has some experience seeing this before. I am worried and think I am just going to take her in. My husband seems to think its just because she is concentrating so hard, and has just this month really starting to stretch and use her arm muscles in this way.

Advise, or experience please.