It may be due to the technique for 半身痳醉 is much higher. If you are in urgent, they will only call the on-call doctor (most likely is MO i.e. junior doctor),full body 痳醉 is much quicker and easier. In the same sense, there is no epidural provided after office hour in public hospital as all the senior doctor are gone home la. In private hospital, you can choose 半身痳醉 even if you are urgent.
Chunchunmama,
Don't worry, as I know, in TMH if you pay around $200 (I forget the exact amount), they will help you to take a BB's photo in the delivery suite, They will provide you an album (like the private hospital one) and a VCD la.