The series, based on the 1989 movie, was due to start later this month, but TV bosses have been forced to move the launch to the end of September due to a "medical evaluation" Tierney is undergoing.
Tierney is best known for playing Dr. Abby Lockhart on hit medical series ER for 10 years, for which she received an Emmy nomination.
It is the second time Parenthood's filming schedule has been pushed back -- in April the pilot was put on hold for two days after NBC's vice president Nora O'Brien died unexpectedly on the set of the show in California.
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