I think scifigal2k's schedule is the way to go, unless there have been new guidelines issued that change that.
Generally, you keep taking it until around the beginning of the third trimester, then hold off until after the delivery/c-section (at least I think that's the standard recommendation).
I'll try to look through my conference notes and see what they said (since I'm a guy who's not going to have kids, the pregnancy presentations are my chance to day-dream
)
If I'm wrong about
any of this, someone please feel free to correct me.
As for your concerns about
effectiveness, there is data that shows that stopping it and re-starting can increase the odds of losing effectiveness and also data that show an increase in risk of developing a reaction to it, as well as developing antibodies to it. But with only missing one or two doses, those risks aren't too bad, and it's what is necessary to prevent the baby from being immunosuppressed.