Explainer: what is placenta?

An incredibly complex and important organ in its own right, the placenta is only found in mammals. And how it functions has the potential to have profound effects on the lifelong health of the developing foetus.

The placenta exists solely during pregnancy, and plays a crucial role in nurturing and protecting the foetus throughout gestation. It’s connected to the foetus via the umbilical cord and attached to the wall of the womb, allowing for essential exchanges of nutrients, gases and waste with the mother’s circulation.

Placenta is composed of maternal and foetal parts, which are known as the basal and chorionic plates, respectively. Nutrients are exchanged through the maternal blood entering the foetal section, but maternal and foetal blood don’t actually mingle; they’re separated by arteries and capillaries.

Interestingly, the placenta has a gender matching that of the foetus, indicated with the presence of either XX or XY sex chromosomes. But placental sex is not used to test fetal gender as that would require invasive surgical tests and add unnecessary risk to the pregnancy.

In the beginning

The placenta begins developing once the embryo is implanted into the wall of the uterus. During the nine months of pregnancy, it increases in size and performs several vital functions. It regulates the exchange of nutrients for foetal growth and development, the exchange of gases including oxygen and carbon dioxide, and hormone secretion.

It also protects the foetus from toxins and infections as well as the mother’s immune system, which would otherwise regard it as a foreign invader. This is a critical aspect of placental physiology; if the mother’s immune system rejects the foetus, it will spontaneously abort.

In order to prepare the developing foetus for the world it will inhabit after pregnancy, the placenta is very sensitive to the mother’s environment. It’s able to adjust its functions in response to external cues, such as the mother’s diet or environmental pollutants, which can then alter foetal development.

The placenta begins developing once the embryo is implanted into the wall of the uterus.Louise Woodcock/Flickr, CC BY-NC

Maternal diet plays a major role in foetal development; studies show eating a balanced diet of fruit, vegetables, and lean meat helps reach a good birth weight. But exposure to pollutants, such as car exhaust fumes, can have a negative impact and may increase the risk of the child developing asthma.

Permanent changes to the developing foetus’s physiology during development is known as foetal programming. And variations in the development of organs and systems within the foetus may increase lifetime susceptibility to cancer, heart disease, allergies and other diseases.

The mechanisms underlying these susceptibilities are incredibly complex and we’re only now beginning to understand them. One of them is epigenetics, which changes foetal gene expression, altering the physiology and functioning of the foetus throughout life.

What can go wrong

Placental disorders can cause serious health complications during pregnancy for both the foetus and mother. They can result in abnormal foetal development, growth restriction, malformations, miscarriage or stillbirth, and may even endanger the mother’s life.

Because the placenta keeps forming throughout pregnancy, abnormalities in its structure and implantation into the uterine wall can happen at any time. Placental abruption, for instance, occurs in approximately one in a 100 pregnancies. Abruption is either the partial or full detachment of the placenta from the uterine wall. And it can deprive the foetus of oxygen and nutrients, potentially leading to preterm birth or stillbirth.

One of the most common disorders of pregnancy is pre-eclampsia, which occurs in 3% to 7% of all pregnancies; it’s the leading cause of maternal health complication and death. Characterised by high blood pressure and protein in the urine, pre-eclampsia can lead to permanent vascular and metabolic damage in the mother.

The exact cause of the disorder is unknown, but it’s thought several factors including poor diet, high body fat, a history of high blood pressure and genetics may all play a role. Abnormal placental development and function is thought to be another major contributing factor.

If left untreated, pre-eclampsia can develop into eclampsia, which is characterised by cerebral fluid build-up and seizures. Once the placenta is removed, pre-eclampsia and eclampsia end.

Out of the several thousand mammalian species, humans are among only a handful that don’t regularly consume the placenta.Kom bo/Flickr, CC BY

The placenta can become infected by bacteria, viruses or parasites which can lead to abnormal foetal development, preterm birth or foetal death. This occurs mostly in developing countries such as Africa, where the malaria parasite contributes to 100,000 deaths annually as a result of severe foetal growth restriction.

Finally, cancer of the placenta, known as choriocarcinoma, occurs in approximately one in 20,000 to 40,000 pregnancies. This cancer usually spreads to lungs and while it can be life-threatening, the cure rate is over 90%. It’s very responsive to chemotherapy, which is given after the baby is born.

Cultural significance

The placenta has little cultural value in Western countries; it’s often unrecognised by parents as being fundamental for a healthy and successful pregnancy. So, it’s usually discarded after childbirth.

But some other cultures hold great respect for this uniquely temporary organ, and have the mother eat it, in a practice known as human placentophagy. According to traditional Chinese medicine, for instance, the placenta is thought to rejuvenate the body after childbirth.

This practice has recently become more popular in Western culture but remains highly controversial, mostly due to the cannabalistic nature of the act. There are few scientific studies examining the benefits of placentophagy, but it’s worth noting that out of the several thousand mammalian species, humans are among only a handful that don’t regularly consume the placenta.

Different cultures hold a variety of beliefs about the placenta. Indonesian and Malaysian cultures consider the placenta to be a sibling of the newborn, for instance. And, in China, it’s thought to be its first and finest clothing. They all have a deep reverence and appreciation for the placenta and its ceremonial role in the birthing event.

The placenta plays a critical role in pregnancy, foetal development and health throughout life. It may only be a temporary organ, but plays some of the most important roles in sustaining early life.

Astrud Tuck receives funding from the Channel 7 Children's Research Foundation.