Mei Xiang was artificially inseminated at the end of April and had abandoned all attempts by zookeepers to conduct ultrasounds, until Wednesday. Changes in her uterus were detected and that's when the fetus was discovered.
The zoo is being very cautious in its excitement, as it is still very possible that Mei Xiang could reabsorb or miscarry the fetus. She is, however, displaying signs of preparing for her child, and if she does keep the pregnancy, it's possible she could give birth as early as next week or the beginning of September.
Despite the fact that Mei Xiang is pregnant in captivity, it's still pretty incredible that we're able to view her potential new life, just like we can as humans.