A panda at Washington DC's National Zoo who is tending to her squealing newborn cub also gave birth to a stillborn cub.
That cub was not fully formed and was never alive outside the womb, a zoo official said.
Mei Xiang gave birth to the motionless cub on Saturday night after giving birth to its live twin the night before, zoo spokeswoman Pamela Baker-Masson said.
The mother groomed her stillborn cub for 17 minutes before letting it fall to the floor, she said.
The mother still has not let zoo staff get close enough to get a good look at her live cub, but it was squealing throughout and appears to be doing well, Ms Baker-Masson said.
The zoo began performing a post-mortem on the stillborn cub that they hope will tell them why the cub stopped developing and died in-utero, she said.
When experts do get to check out the live cub, they will try to listen to its heart and lungs, record its weight and collect a DNA sample.
Brandie Smith, the zoo's curator of mammals, said she is "cautiously optimistic" about that cub's health.
She compared the planned examination to a racing car pit stop, a fast and highly choreographed check-up before reuniting mother and cub.
The live cub was the 15-year-old panda's third. Mei Xiang gave birth to a cub last year that died after just six days.