I forget where you're located. Yes, as blacksnake says, the pasture is cleaner than a dirty barn stall. It's also difficult to confine them, especially if they are not used to being locked in a stall, and this will be stressful to them.
It is helpful, especially with heifers, to keep them in a smaller pasture/enclosure so that you can keep an eye on them and assist if needed, and that way you're not stumbling around a large pasture in the dark to find a heifer in labor.
Hope that all goes well for you and them and that they calve safely!