The placenta contains high levels of prostaglandin which stimulates involution (an inward curvature or penetration, or, a shrinking or return to a former size) of the uterus, in effect cleaning the uterus out. The placenta also contains small amounts of oxytocin which eases birth stress and causes the smooth muscles around the mammary cells to contract and eject milk.. There is no scientific evidence, however, that eating placenta provides these hormonal effects.
There is also a school of thought that holds that placentophagy naturally occurred to hide any trace of childbirth from predators in the wild. Most placental mammals participate in placentophagy, including herbivores; exceptions include Pinnipedia, Cetacea, the camel and (in most cases) the human. Placentophagy has been observed in Insectivora, Rodentia, Chiroptera, Lagomorpha, Carnivora, Perissodactyla, Artiodactyla (with the camel as a noted exception), and Primates. Marsupials, which are an order of metatherian (pouched) mammals, resorb rather than deliver the placenta, and therefore cannot engage in placentophagia; they do, however, vigorously lick birth fluids as they are excreted.
Some research has shown that ingestion of the placenta can increase the pain threshold in pregnant rats. Rats that consumed the placenta experienced a modest amount of elevation of naturally-occurring opioid-mediated analgesia. Endogenous opioids, such as endorphin and dynorphin, are natural chemicals, related to the opium molecule, that are produced in the central nervous system. Production of these endogenous opioids is increased during the birthing process. They have the ability to raise the threshold of pain tolerance in the mother. When coupled with ingested placenta or amniotic fluid, the opioid effect on pain threshold is dramatically increased. Rats that were given meat instead of the placenta showed no increase in the pain threshold. There have been no scientific studies which show that placentophagy enhances analgesia in humans.
Human placentophagy 
Although the placenta is revered in many cultures, there is scarce evidence that any customarily eat the placenta after the newborn's birth. Despite an urban legend that the Basque people ate placentas, there is no evidence that they ever did, and the Euskara word for placenta, karena, is not etymologically related to their word for cannibalism, gizajana.
Those who advocate placentophagy in humans believe that eating the placenta prevents postpartum depression and other pregnancy complications. Obstetrician and spokesperson for the Royal College of Obstetricians and Gynaecologists Maggie Blott disputes the post-natal depression theory, stating there is no medical reason to eat the placenta; "Animals eat their placenta to get nutrition - but when people are already well-nourished, there is no benefit, there is no reason to do it." On the other hand, American Medical anthropologists at the University of South Florida and UNLV, surveyed new mothers, and found that about 3/4 had positive experiences from eating their own placenta, citing "improved mood", "increased energy", and "improved lactation". 
Human placenta has also been an ingredient in some traditional Chinese medicines, including using dried human placenta, known as "Ziheche" (simplified Chinese: 紫河车; traditional Chinese: 紫河車; pinyin: Zǐhéchē), to treat wasting diseases, infertility, impotence and other conditions.
See also 
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- Mark B. Kristal (2 February 1980), "Placentophagia: A Biobehavioral Enigma", Neuroscience & Biobehavioral Reviews 4: 141–150, archived from the original on September 30, 2007, retrieved 2007-12-12
- Mark B. Kristal (May 2012), "Placentophagia in Humans and Nonhuman Mammals: Causes and Consequences", Ecology of Food and Nutrition[dead link]
- Mark B. Kristal (1991), "Enhancement of Opioid-Mediated Analgesia: A Solution to the Enigma of Placentophagia", Neuroscience and Biobehavioral Reviews
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- Bawany, Afsha (February 27, 2013). "Steamed, Dehydrated or Raw: Placentas May Help Moms’ Post-Partum Health. UNLV anthropology survey examines why women consume their placentas after childbirth.". UNLV News Center. Retrieved March 25, 2013.
- J. Selender, A. Cantor, S. Young, and D. Benyshek. "Human Maternal Placentophagy: A Survey of Self-Reported Motivations and Experiences Associated with Placenta Consumption". Ecology of Food and Nutrition. Retrieved March 25, 2013.
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- Tierra, Lesley; Tierra, Michael (1998), Chinese traditional herbal medicine, Twin Lakes, WI: Lotus Light Pub, pp. 225, ISBN 0-914955-32-2
- "Channel 4 rapped for serving placenta". BBC. Retrieved 17 December 2012.