How out of control do your gallstones have to be before they take your baby early and take the gall bladder?

I am 36 weeks pregnant. A month ago my doctor told me I have gallstones, and gave me a prescription that's supposed to prevent the attacks. It worked for a while, but tonight I had another attack. It was awful, and it lasted an hour. When I saw my doctor two weeks ago he said if this gets out of control, we will have to c-section the baby and take your gall bladder out. I guess my question is what is out of control? The medicane obviously isn't going to stop every attack.