In theaters on November 18, 2011

Plot synopsis from Wikipedia
The newfound married bliss of Bella Swan (Kristen Stewart) and vampire Edward Cullen (Robert Pattinson) is cut short when a series of betrayals and misfortunes threatens to destroy their world. After their wedding, Bella and Edward travel to Rio de Janeiro for their honeymoon, where they finally consummate their marriage . Bella soon discovers she is pregnant, and during a nearly fatal childbirth, Edward finally fulfills her wish to become immortal. But the arrival of their remarkable daughter, Renesmee, sets in motion a perilous chain of events that pits the Cullens and their allies against the Volturi, the fearsome council of vampire leaders, setting the stage for an all-out battle.

See the Breaking Dawn Official Website for more details