Pandas are notoriously poor breeders, in the wild and especially in captivity. For a brief review of the difficulties with panda pregnancy, see this article from ScienCentral News. Excerpted without permission:
The scant patterns that have been found indicate that a pregnant panda’s urine hormones track exactly to a non-pregnant panda’s, except for a four- or five-day period about 30 days before delivery.
The change in hormones is thought to correspond to the implantation of the panda fetus, another mysterious event. Called variable delayed implantation, this phenomenon of pandas means that the embryo floats freely in the uterus until, researchers think, conditions for a successful birth are deemed to be favorable and the fetus implants in the wall of the uterus late in the three-to five-month pregnancy. A human embryo implants within days of fertilization.