It is impossible to predict when a giant panda will give birth even when the date of conception is known. It is because giant panda gestation is very variable, from three to five and a half months. Among captive animals at Beijing Zoo, the time between mating and birth varies from 122 to 163 days, with an average of about 140 days, although cubs have been born after as little as 96 days or as much as 168 days. This suggest that when the female has been fertilized the embryo does not begin to develop immediately, a theory that is supported by studies on captives which show that hormones usually associated with pregnancy are not detectable until 5-6 weeks before birth.

The delayed implantation of the embryo is an adaptation found in a number of other temperate species such as roe deer and bears. It allows animals to adjust the interval between mating and lactation occur at a time which suits the female. It has no obvious advantages for giant pandas. Female give birth during August and September which foraging mostly on nutritious bamboo leaves, but there is no obvious reason why this birth season could not be achieved by matings in June and July followed by a normal pregnancy. A famous researcher, George Schaller argues that delayed implantation might give the species flexibility, which could have allowed the giant panda to adapt to a variety of conditions in its larger historical range. However, even in its present range the giant panda utilizes a wide variety of bamboo species which shoot in different seasons, and yet there is little seasonal variation in mating or birth times. Alternatively, the mechanism might allow females to respond to changes in the quality of their food, so that if food quality remains low during the autumn, the embryo might fail to implant. This would save the female the energetic drain of an undesirable pregnancy, but there is no evidence that birth rates were reduced following the bamboo flowering in Wolong, when presumably the pandas were under nutritional stress.

The fertilized egg develops to the blastocyst stage, a ball of cells no bigger than a pin-head, but then, instead of implanting in the womb as it does in most mammals, it remains free-floating. Only when an increased production of progesterone is triggered the womb thickens and the blastocyst implants.

The giant pandas are able to predict in advance when optimal conditions will occur so that implantation can take place at the appropriate time are at an advantage over those that cannot. The sort of external cues that pandas may use to predict favourable conditions in a stable environment are day length, temperature and rainfall, although the latter is less reliable. Secondary, less accurate, cues include fluctuations in the availability of nesting sites, ground cover and the abundance of predators. If conditions are favourable, the blastocyst will implant but if food is scarce and the female's nutrition is below par, then the blastocyst will continue to float free in the womb. This explanation certainly fits with the fact that the period of delayed implantation in giant panda occurs during the lean season when, both in terms of quality and calories, panda food falls below par. One must assume that if food conditions were to remain poor, the blastocyst would not implant and the female would be saved from the energy drain of pregnancy. Judging from the very undeveloped state of a newborn giant panda, development from the blastocyst stage to birth probably only takes about six weeks which means, given the gestation period, that the delay in implantation varies from about one and a half to four months.

Pregnant giant pandas have to build a nest. They seek out suitable den sites - caves, dense bamboo thickets or hollows at the bases of trees - and line them with any material that will keep out the damp and cold and provide a comfortable cushion. Wood chips, birch and fir saplings, and branches of rhododendron or bamboo are the necessary materials. Because dens have to be roomy enough for the female to sit upright comfortably, a tree must be at least 90-100 cm in diameter at breast height and have a large hollow. Such trees are at least 200 years old and are uncommon, especially in young or logged forests. The scarcity of these premier den sites may actually be a limiting factor on giant panda survival than the quality and supply of bamboo.

Once the nest has been prepared a few days before birth, the female loses interest in food and water. Her nipples and vulva become swollen and she squats or sits to give birth. Giant panda labour should be among the easiest of all mammals because the size of the infant is very tiny compared to its mother's size. When the head of the young appears, she may help the rest of the infant to emerge by gripping it between her jaws and pulling gently. The female will nurse it immediately, presses it against her breast with a forepaw.