Cast member Maura Tierney arrives for the premiere of the film "Semi-Pro" at the Mann Village Theater in Los Angeles, February 19, 2008. [Agencies]

LOS ANGELES- NBC's new fall drama series "Parenthood" has been pushed to midseason because of an unspecified illness that has befallen Maura Tierney, one of its stars.

The series, a contemporary reimagining of the 1989 feature, had been scheduled to begin filming July 27.

The production start has been postponed for eight weeks to late September "due to medical evaluation that (Tierney) is undergoing," NBC said. "We are unable to release further details and ask that you respect her privacy at this time."

"Parenthood" revolves around four siblings (Peter Krause, Tierney, Erika Christensen, Dax Shepard) and their parents (Bonnie Bedelia, Craig T. Nelson).

The series had been slated to premiere September 23 and run in the 8 p.m. Wednesday hour.

With "Parenthood" pushed, the new drama "Mercy," which was to take over the 8 p.m. Wednesday slot in midseason, will launch in the fall. The cast and crew of the medical drama have been summoned to start work on the series early to have it ready for a September debut.

It was not yet clear whether "Mercy" will air at 8 p.m. Wednesdays as NBC is re-evaluating its fall scheduling plans that now will include the simultaneous launch of two new medical dramas, "Mercy" and "Trauma."

Meanwhile, the delay in production will give "Parenthood" writers more time to work on scripts.

This is the second time filming on "Parenthood" has been put on hold following unfortunate circumstances. In April, production on the pilot was shut down for two days after NBC vp drama Nora O'Brien died unexpectedly on the Berkeley, Calif., set of the show.

Tierney is perhaps best known for her 10-year stint as Dr. Abby Lockhart on "ER," for which she earned an Emmy nomination. She also starred on another NBC series, the comedy "NewsRadio," and has appeared in several features, including "Insomnia" and "Baby Mama."

"Mercy" chronicles the lives of the staff at Mercy Hospital through the eyes of three nurses (Taylor Schilling, Michelle Trachtenberg and Jaime Lee Kirchner).