Difficulty urinating after contractions have begun

[oysterpots]

Is it normal to feel like you're only emptying about half of your bladder when you've gone into pre-labour?

I had real trouble going to the loo with DS i felt like i was bursting but when I sat there the pain increased loads and i couldn't get up again. I guess its all due to pressure from the baby's head.

I don't think it matters how much you pee, as long as you keep peeing. The baby's head and the contractions may affect how easily you pee, IIRC. If you can't pee at all then you must go to the hospital.