Zoo officials said today they have had the 13-year-old mother on round-the-clock monitoring since Monday because she was exhibiting hormonal and behavioral changes associated with pregnancy.

Giant panda sometimes act pregnant even when they are not.

This would be Lun Lun's third cub. Her previous two cubs are 4-year-old Mei Lan and 2-year-old Xi Lan.

Giant panda gestation averages 135 days but can range from 83 to 197 days. Lun Lun was inseminated with sperm from her partner, Yang Yang, earlier this year.