WASHINGTON (Reuters): National Zoo staff are keeping a close eye on giant panda Mei Xiang’s newborn twins, given the cubs’ fragile state just days after their birth, the zoo’s lead veterinarian said on Monday. “The cubs are doing pretty well. These early days everything’s still touch and go, so we’re watching them closely but we’re very hopeful right now,” Dr. Brandie Smith told CNN. Mei Xiang, a star tourist draw, took staff by surprise on Saturday by giving birth to the twins about four-and-a-half hours apart after being artificially inseminated.

Giant pandas are among the world’s most endangered species. Smith said the new cubs are so tiny and fragile that problems could still arise, anything from a congenital defect to difficulties with the mother panda’s ability to care for the two new small babies, each still fur-less and about the size of a stick of butter. “Almost anything could go wrong in these first few days,” she told CNN’s “New Day” program. Smith is the zoo’s associate director for animal care sciences, overseeing care for all of its mammals.