Smithsonian National Zoo / AP
In this image from video provided by the Smithsonian National Zoo, Mei Xiang gives birth to a cub two hours after her water broke Friday, Aug. 23, 2013, at the National Zoo in Washington. The zoo has been on round-the-clock panda watch since Aug. 7, when Mei Xiang began showing behavioral changes consistent with a pregnancy or pseudopregnancy.