"If a woman has cancer and she's pregnant, is she allowed to take cancer-curing drugs even though they will probably harm her baby? Ok it's a bit of an extreme comparison and therefore a bit sneaky because cancer can affect even those who make good lifestyle choices, whereas smoking is taken up voluntarily, but the principle remains. You either believe the life of the mother is paramount or that the baby takes precedence over the mother while in the womb."
It's a bit more complex than that.
Taking a cancer drug may save the mothers life, not taking it may at least save the baby. Or either way, both still may sadly die.
Smoking in pregnancy is not the same. The cancer drug is hopefully a lifesaver, something smoking will never be. In this case the life of the mother is not paramount because she won't die if she stops smoking. At worst, she and/or her baby could die from a smoking related cause if she carries on.
I developed an infection in my placenta while I was pregnant and the doctors were forced to make the decision to save my life knowing it would cost my daughter hers (she was born too prematurely to survive). Previously they had been doing everything they could to ensure I didn't go into early labour (I had lost my mucus plug, my cervix was opening and eventually my waters broke. An infection was almost certainly a given but they still tried so hard until I became seriously ill very quickly and the infection was confirmed.
Had it been possible to save her life knowing it might cost me mine then that would have been my choice, no doubt about it. But the infection would have killed us both before we reached viability so it was never an option to wait. I still tried to argue the case for waiting for viability anyway and my doctor told me they could have me declared mentally unfit to make my own decision and ask my DH to make it for me if I didn't agree to inducing labour immediately. They said the placenta would kill us both if it wasn't taken out soon.
I'm pro-choice, I would have said that I believe that until the baby is born the mother takes priority in a life or death situation...except in my own case when I would have given my life and the lives of everyone else in the room with us to save hers. There would have been no excuses and no justifications from me if we'd had even the slimmest chance of saving her life rather than risking it or ending it.
This isn't an immediate life or death situation for the mother, so comparing it to a genuine life or death situation requiring cancer treatment isn't really fair.