As a nurse who works on the women's floor. We try for whatever is best and can get. If we put it in your hand I usually try for your non dominate hand there are some spots on the forearm and arm that are good veins. It all depends on how hydrated and how well your veins are to find. Iv's in the AC (where they usually draw blood at the bend of your elbow) are usually easy except it's an easy vein to get kinked and make iv pumps go crazy if you bend it just right.

I think it's where ever they find the best vein.. I had it in my hand with my first and in my arm with my second

They can put it in your forearm. As a nurse i do not like sticking hands and especially being in labor and having baby I want you to be able to wash your hands and hold baby, feed baby etc without something in your hand. And as an ambidextrous person I couldn't do that to someone unless there were no way to prevent it.

Any iv I've ever had I have always requested they put it in my arm. My veins are crazy and roll around and blow easy any where else. Just ask. 

I didn't have an IV with my son so maybe it's only certain circumstances they give you one.