Hormones: What to Expect in Giant Panda Pregnancy or Pseudopregnancy
Following Mei Xiang's artificial insemination, Zoo scientists are monitoring her hormone levels to track pregnancy or pseudopregnancy.
As expected, following Mei Xiang's estrus, her urinary estrogens declined to basal with a concomitant rise in urinary progestins. For the giant panda, an initial rise in excreted progestins occurs immediately after the peri-ovulatory estrogen peak. This initial post-ovulatory rise in urinary progestins, although not as robust as the later secondary rise (see below), signals that ovulation occurred.
See hormone profiles.
For pregnant pandas, the onset of the secondary progestin rise is when the embryo finally becomes implanted in the uterine wall and begins fetal development; continued progestin production is believed essential for continued fetal growth and maintenance.
The phenomenon of delayed implantation—or the delay from when conception occurs in the immediate post-ovulatory period until implantation in the uterine wall—occurs in all bears. The delay is believed to be due to insufficiency of the corpus luteum (CL), the ovarian structure that produces progesterone. When the ovary is later exposed to increasing concentrations of a pituitary hormone called prolactin, the CL becomes reactivated. This results in a dramatic increase in CL-derived progesterone production, which then has a permissive effect, helping to create a uterine environment conducive to embryo implantation, should an embryo be present (that is, if the female is pregnant).
Delayed CL reactivation in bears is obligate once ovulation occurs, whether or not a female conceives, and this is why some non-pregnant females are classified as pseudopregnant. Females have no way of assessing their own pregnancy status, so pseudopregnancy probably functions as the hormonal equivalent of an insurance policy to protect pregnancy, should a female be carrying an embryo awaiting implantation.
Currently there is no pregnancy detection method available for the giant panda, however we will track the urinary progestin concentrations as a valuable sentinel measure because when levels decline to baseline (after the secondary rise), this either signals an impending birth or, in the case of a false pregnancy, indicates that the pregnancy watch is no longer necessary.