The trailer released by Universal Pictures shows Zellweger's titular character, Bridget Jones, finding herself pregnant as she must determine if the father is her ex-boyfriend Mark Darcy or new flame Jack Qwant, portrayed by Firth and Dempsey respectively.
"In an unlikely twist, she finds herself pregnant, but with one hitch...she can only be 50 percent sure of the identity of her baby's father," the trailer's description states.
Firth's and Dempsey's character vie to be the father of the unborn child as the trio embark on a series of comical adventures, including attending a birthing class together.
The third installment in the Bridget Jones franchise is directed by Sharon Maguire and also stars Academy Award winner Emma Thompson.
Bridget Jones's Baby is set to release in theaters on Sept. 16.