Alice, with her ability to see the future, quickly realizes that Bella is pregnant and calls Bella during her honeymoon. Although she has become Bella's confidante, Alice is not very involved in the events surrounding Bella's pregnancy, because the fetus incapacitates her mental gifts and gives her a headache.
After she "sees" the Volturi army approaching, she disappears with Jasper, leaving everyone to believe that they deserted the Cullens to save their own lives. But Alice leaves a note for Bella that leads her to J. Jenks, the lawyer who forges fake IDs and documents for Renesmee and Jacob in the event that they have to go on the run and hide from the Volturi. To everyone's surprise, Alice and Jasper return during the face-off between the Cullens and the Volturi with a young male vampire hybrid named Nahuel, who confirms that Renesmee, a being just like him, poses no danger to vampires or the Volturi. The evidence he brings forth plays a vital role in forcing the Volturi to retreat.
For more info on Alice, check out her "Character Analyses" in the other books of the Twilight series on Shmoop.