As the public learned soon after Mei Xiang gave birth, though, Bei Bei's twin didn't make it through its earliest hours on Earth, a sadly common event in panda twin births.
Any cubs are minor miracles to begin with. Giant pandas in captivity are not exactly prolific multipliers of their species. Females are in estrus for very short windows of time just once per year, with only about a week's time where they will be receptive to mating, and only about a day and a half where they are fertile.
Mei Xiang, for her part, has been down this road before. It was only last year that her daughter Bao Bao made her debut.