Zookeepers at Washington D.C.'s National Zoo are abuzz with excitement as they wait to see if Mei Xiang the panda is pregnant. She's experiencing a rise in urinary progesterone, which scientists say, could mean she's pregnant. They've also seen her building a nest. Ultrasound images are inconclusive - Panda fetuses don't develop until the final weeks of gestation. Visitors are thrilled with the news.