(DC)- The National Zoo is closing its Panda House because it believes its giant mama panda might be pregnant. Zoo experts say Mei Xiang has started to show changes in her behavior this week that may indicate she’s pregnant. Zoo keepers say her levels of progesterone are rising, which is usually a sign. Mei Xiang has also started building a small nest of shredded bamboo in her den. The only way to definitively determine if a giant panda is pregnant before she gives birth is to detect a developing fetus on an ultrasound. Veterinarians have been conducting regular ultrasounds to track Mei Xiang’s reproductive tract, but have not seen anything just yet.