The giant panda is one of the world's most endangered species, with an estimated 1,900 in existence.
America welcomed its first panda twins in 26 years in July at Zoo Atlanta. The twins were the first for Lun Lun, who has two other offspring at that zoo, and were the product of artificial insemination as well. Their father is Yang Yang, also a resident at Atlanta's zoo.
National Zoo experts began watching Mei Xiang a couple of weeks ago. The anticipation peaked when she started cradling objects.