EXCLUSIVE: Reese Witherspoon/Jack Nicholson Movie Delayed After Actress Gives Birth

Production in a major new movie, starring Oscar winners Reese Witherspoon and Jack Nicholson, was held up for a day after an actress in the film -- currently called 'Untitled James L. Brooks Project' -- unexpectedly went into labor prematurely at her hotel, and had to be rushed to the hospital, RadarOnline.com has learned exclusively.

Actress Kathryn Hahn (Crossing Jordan) -- who plays a pregnant secretary in the film -- is pregnant in real life. She went into labor after her water broke during filming Saturday.

On Monday morning, after a grueling two days of labor, Hahn reaped the fruits of her labor: Mae, a 5-pound girl.

Though her due date was originally projected as September, the unexpected birth doesn't throw a monkey wrench into the plans of the film's producers, who expected Hahn to give birth after filming wrapped -- the actress had only one day left to film, and will finish the rest of her scenes when she has recovered.

The film also stars Paul Rudd and Owen Wilson.