Maura Tierney, Michelle Trachtenberg

The start of production on NBC's Parenthood, which was to premiere this fall, has been postponed for eight weeks "due to a medical evaluation that valued cast member Maura Tierney is undergoing," the network says in a statement.

"The top priority for NBC, Universal Media Studios and Imagine Television is the well-being of Ms. Tierney," the statement said. "We are unable to release further details, and ask that you respect her privacy at this time."

As a result of the production delay, Parenthood will now premiere at midseason.

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To fill the fall void, NBC has summoned to work the cast and crew of the medical drama Mercy, which wasn't scheduled to debut until midseason. Mercy will now premiere in the fall, though it's unclear if it will occupy Parenthood's Wednesdays-at-8 slot.

With Mercy starting production months ahead of schedule, there is the question of whether one of that series' leads, Michelle Trachtenberg, now might not be available for a planned limited run on Gossip Girl.

While neither the CW nor Trachtenberg's rep immediately responded to TVGuide.com's requests for comment, production sources tell TVGuide.com that at this time, no scheduling conflict is foreseen.