Original cartoon by Alexandra Martin
Source: Original cartoon by Alexandra Martin

The placenta is a defining feature of the main group of mammals, to which humans belong. It is the interface across which mother and fetus exchange nutrients and waste products during pregnancy. The name comes from plakoenta, Greek for “flat cake”—a prototype pizza if you will. In the vast majority of species mothers devour the placenta soon after birth (placentophagy). As Benjamin Tycko and Argiris Efstratiadis aptly noted in a Nature commentary, they “have their cake and eat it too.” And the really unusual thing is that it is not just naturally carnivorous mammals that eat the placenta. Herbivores such as cattle, goats and horses, which normally shun animal food of any kind, devour the placenta just as avidly. Few exceptions exist, and curiously most are carnivorous, notably pinnipeds (seals and sea lions) and cetaceans (dolphins and whales). Among herbivorous mammals, camels and their kin are the only reported exception. Perhaps eating raw meat covered in sand is too much to ask? Regardless of their dietary habits, all nonhuman primates eat the placenta. Indeed, extensive experience with captive breeding of primates taught me that failure to eat the placenta is a warning sign that maternal care might not kick in properly. Humans contrast starkly with all other primates in having no inclination to eat the placenta after birth. When and why did change occur?

 Fir0002 at English Wikipedia.
1) Rat eating placenta; (2) Goat eating placenta
Source: (1) Chuck F [Public domain], via Wikimedia Commons; (2) Transferred from en.wikipedia to Commons by Fernandopascullo. Author: Fir0002 at English Wikipedia.

Why eat the placenta?

The tricky part of placentophagy is that a mother must be programmed to eat the placenta and chew the umbilical cord but stop before harming the newborn baby. So why do most mammals eat the placenta despite the potential danger to the offspring? One long-favoured explanation is that this helps avoid soiling the home base and/or detection of a birth by predators. Yet even top land-living carnivores, which face no substantial predation threat, eat the placenta.

Public domain via Wikimedia Commons. No machine-readable author provided; Habj assumed, based on copyright claims.
Human placenta shortly after birth, showing the side with the umbilical cord facing the baby.
Source: Public domain via Wikimedia Commons. No machine-readable author provided; Habj assumed, based on copyright claims.

Alternative possibilities are that placentophagy benefits the mother’s health or triggers maternal care. For starters, the average human placenta weighs about a pound and it seems wasteful to discard this potential source of nutrients. In addition to trace elements and minerals, it contains a score of hormones and hormone-like prostaglandins, which stimulate the womb to return to its non-pregnant state. The hormones include oxytocin, a multi-purpose agent that reduces stressful effects of birth and stimulates contraction of muscles around mammary cells in the breast, triggering milk ejection.

For some 40 years now, Mark Kristal has conducted experiments with laboratory rodents, searching for beneficial effects of placentophagy as his primary research focus. In 1991, his team reported two important findings: First, increased contact associated with eating the afterbirth and swallowing amniotic fluid promotes mother-infant interaction. Second, devouring the placenta and amniotic fluid enhances pain relief (analgesia) by triggering release of morphine-like opioids in the brain. Kristal named the active substance in placenta and amniotic fluid Placental Opioid-Enhancing Factor (POEF). He and his colleagues found that POEF enhances pain relief in male as well as female rats, whereas ingestion of liver from pregnant rats (as a control) has no effect. They also discovered POEF activity in afterbirth material from dolphins and humans, both of which lack placentophagy. In a 2012 review, Kristal and colleagues noted that human birth is prone to several problems, including postpartum depression, bonding failure and hostility toward infants. The human placenta may contain substances that would diminish these problems, but this has not been scientifically tested. At present there is no good evidence that human mothers miss out by not eating the afterbirth.

//wellcomeimages.org/indexplus/obf_images/a9/25/1c9446ae9d5d4a0fe3677c8d9203.jpg. Photograph of Bali placenta burial site provided by Lu Yao, with permission.

Illustrations of the custom of placenta burial reported for half of human societies. Left: Ming dynasty painting taken from the supplement to Buyi Lei Gong’s Guide to the Preparation of Drugs (1591 edition). Right: Stone with flowers in front of a house in Bali indicating burial of a placenta, in the belief that it will act like an angel to protect a child of either sex.

Source: Ming dynasty painting from Wellcome Images, a website operated by Wellcome Trust, a global charitable foundation based in the United Kingdom. http://wellcomeimages.org/indexplus/obf_images/a9/25/1c9446ae9d5d4a0fe36.... Photograph of Bali placenta burial site provided by Lu Yao, with permission.

What do people do with the placenta?

The placenta has special cultural significance in many human societies, but it is rarely eaten after birth. In a 1945 review based on the Cross-Cultural Survey at Yale University, American anthropologist Clellan Ford reported that the placenta was in fact buried in half of the human societies documented. A more recent analysis of the updated survey data by Sharon Young and Daniel Benyshek, reported in 2010, examined cultural beliefs and disposal practices involving the placenta in 179 human societies. Apart from two questionable instances, cultural traditions involving placentophagy were conspicuously absent. For 109 societies (61%), one or more specific procedures for proper disposal of the placenta were identified. Agreeing with Ford’s original findings, by far the commonest practice was burial, documented for 93 societies (55%). In a few societies, the placenta was placed in a tree, incinerated or discarded in or near a lake. Of the 109 societies with a designated placenta ritual, one or more connections with some form of magic or supernatural influence were identified in 67 (62%). The most frequent belief (55% of cases), is that the way in which the placenta is handled can serve to either foretell or alter the future.

People who eat the placenta

Although placentophagy is generally absent from human societies, some believe that eating the placenta has benefits such as alleviating postnatal blues and other pregnancy complications. Consumption of the human placenta has been reported for Hawaii, Mexico, certain Pacific islands and China. It is also an ingredient in some traditional Chinese medicines, although not just for new mothers. According to William Ober, Li Shih-chen’s Great Pharmacopoiea (1596) recommended a mixture of human milk and placental tissue for a rather vague ailment known as ch'i exhaustion.

Information and images provided by Lu Yao, with permission.
Dried human placenta (紫河车, zi he che), a traditional, general-purpose Chinese medicine prepared using a combination of boiling with spices, steaming and drying.
Source: Information and images provided by Lu Yao, with permission.

As a relatively recent trend, in USA and Europe there has been mounting interest in possible beneficial properties of ingesting the placenta. In one prominent example, in 2006 American psychologist Jodi Selander founded the website PlacentaBenefits.info to encourage mothers to consume the placenta. She developed a proprietary method of placenta encapsulation based on procedures in traditional Chinese medicine. Her website states that swallowing placenta capsules speeds recovery after birth and reduces the risk of postnatal blues. Considerable media attention resulted in 2013 when actress January Jones announced that after giving birth she had ingested her placenta as capsules. She felt that this boosted her energy, enabling her to return to work within weeks.

Placentophagy also takes more outlandish forms. Witness Robin Cook’s 2013 digital cookbook 25 Placenta Recipes - Easy and Delicious Recipes for Cooking with Placenta! In fact, William Ober reported learning from a Czechoslovakian medical officer that midwives and obstetrical nurses in Haiphong, Vietnam were eating placentae from patients. After removing the membranous parts, they diced the placentae into small pieces and fried them, usually with onions.

Does processing the placenta destroy potential benefits? It seems highly likely that cooking would, but no information is available. In two 2016 papers, however, Young, Benyshek and colleagues reported findings for human placenta processed for encapsulation. In the first, they detected arsenic, cadmium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, rubidium, selenium, strontium, uranium and zinc. The only noteworthy finding is that for iron the specified capsule intake would provide a quarter of the daily Recommended Dietary Allowance. Average concentrations of potentially harmful elements (arsenic, cadmium, lead, mercury, uranium) were well below accepted toxic levels. In their second paper, Young and colleagues measured concentrations of 16 steroid hormones and melatonin in placenta processed for encapsulation. All but one of the steroids were detected in all placenta samples, albeit mostly at very low levels, while melatonin was detectable only in a third. But a few steroids—notably estradiol and progesterone—were present at levels where physiological effects would be expected.

Why do women not eat the afterbirth?

Humans clearly lack any inclination to consume the raw placenta after birth. In this, we are alone among primates and highly unusual among mammals generally. It is also probable that the last common ancestor of chimpanzees and humans still exhibited placentophagy. A spectacular video sequence from the Durrell Wildlife Conservation Trust (Jersey, Channel Islands) shows an orangutan mother giving birth and then devouring the placenta. Chimpanzees and gorillas naturally behave in similar fashion.

So when and why did placentophagy disappear during human evolution? No relevant fossil evidence exists, so we can only speculate about timing. Was it a gradual reduction leading to eventual loss, or did rapid change occur at some stage? Unfortunately, comparison with other mammals has little to tell us either. No other primate has abandoned placentophagy, so there is no parallel case to guide us. If we forget about camels and their kin, loss of placentophagy in other mammals has occurred only in sea-living species: seals, sea-lions, dolphins and whales. This tells us that loss of placentophagy is linked to major habitat change. Perhaps the switch from a basic diet of plant foods to omnivory with regular meat consumption during human evolution might have been a key factor. An intriguing new hypothesis, proposed by Sarah Young and colleagues in 2012, links our loss of placentophagy to fire use. They suggested that pregnant women would have been regularly exposed to smoke and ash, known to contain toxins. The placenta sequesters certain toxins, which accumulate over the course of pregnancy, so selection might have acted against placentophagy. Because use of fire dates back at most 2 million years and possibly considerably less, this hypothesis would indicate that placentophagy was lost only after our genus Homo emerged.

References

Beacock, M. (2012) Does eating placenta offer postpartum health benefits? British Journal of Midwifery 20:464-469

Cook, R. (2013) 25 Placenta Recipes - Easy and Delicious Recipes for Cooking with Placenta! Seattle: Amazon Digital Services LLC.

Ford, C.S. (1945) A Comparative Study of Human Reproduction. New Haven, CT: Yale University Press.

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