Hi.
A lot of very good information given here. I like the one about Water. swimming for an 18 month old is hard!!! i would suggest speaking to a physio that works with the Halliwick method. Theres so much to be achieved with this therapy and the results are very good.....eg improvement in ballance -because the body falls slower against the water resistance, the brain has longer to react and this in turn has an effect out of water, Muscle lengthening and strenthening, coordination training and a hell of a lot more.
Try this one-in Water your little girl can stand on your knees with light support from you starting with her shoulders under water, then over a period of time slowly raise her out a fraction and allow her during this movement to ballance herself. when you feel her losing it, lower her back down untill she gathers herself again. move nice and slow so that she dosn't panic. at first it's best to support her lightly under the arms. when shes got used to the exercise lower your hands to her waist, then hips, Thighs and knees. When her ballance has improved you can experiment by raising one of your Knees and sinking the other a cm or so to simulate uneven ground.
This is a very usefull exercise that you can do youself but i would go along with everything else thats been written and get a definate diagnosis.
good luck and happy parenthood.
tony