What Is a Placenta?

Medically Reviewed on 1/27/2022
What Is a Placenta?
The placenta is a special organ that develops in the uterus to support the life of the fetus.

During pregnancy, a special organ develops in the uterus to support the life of the fetus; this is known as the placenta. As the embryo implants in the uterus, some cells from the embryo transform into special structures, which eventually develop into the placenta.

One end of the placenta is attached directly to the uterus, and on the other end, it is attached to the fetus through the umbilical cord.

  • The placenta is responsible to provide the fetus with oxygen, nutrients, hormones, and other essential substances through the mother’s blood.
  • It is important for the survival of the fetus.
  • The placenta removes metabolic waste products from the fetus.

After the baby is delivered, the placenta detaches from the uterus and is delivered. Delivery of the placenta is considered the third stage of labor.

The placenta of humans is discoid and hemochorial, which means the shape of the placenta is disc-shaped, and the fetal part of the placenta (chorion) is in direct contact with the blood of the mother.

3 parts of the placenta

The placenta is divided into three layers that include:

  1. Amnion: Amnion is the name given to the innermost placental layers that surround the fetus. Within the amniotic cavity lies the fetus immersed in the amniotic fluid. Amniotic fluid is a clear yellow liquid that allows the proper development of the fetus.
  2. Allantois: Allantois is the placenta's middle layer that is derived from the embryonic hindgut (digestive system). It eventually develops into urachus that drains fetal urine from the urinary bladder. Urachus is a cord-like structure that passes through the umbilical cord.
  3. Chorion: Chorion is the placenta’s outermost layer and comes into contact with the endometrium (lining of the uterus). The chorion grows a dense network of blood vessels and forms a close relationship with the endometrium. The placenta, which is the embryo's primary organ of respiration, feeding, and excretion, is formed by the chorion and endometrium.
    • It is made up of two layers of cells:
      • Cytotrophoblast on the inner side
      • Syncytiotrophoblast on the outer side

4 common types of placental placements

Wherever the fertilized egg implants in the uterus, the placenta attaches and develops.

The placenta can be placed in various locations; however, here are a few of the most common ones:

  1. Posterior placenta: The placenta develops in the back of the uterine wall, where the fertilized egg is attached.
  2. Anterior placenta: The placenta adheres to the front wall of the uterus, and the fetus develops behind it.
  3. Fundal placenta: This occurs when the placenta attaches to the upper wall of your uterus.
  4. Left/right lateral placenta: The placenta adheres to the left or right wall of the uterus.


The first sign of pregnancy is most often: See Answer

4 abnormal placental attachments

Sometimes, the placenta attaches and grows in positions that cause complications during the development of the fetus or may cause complications during vaginal delivery. These abnormal positions are detected while performing routine ultrasound scans.

  1. Placenta previa: A placenta previa pregnancy issue occurs when the placenta attaches low within the uterus, covering all or part of the cervix. The placenta normally connects to the upper region of the uterus, away from the cervix, allowing the placenta optimal blood supply and oxygenation and a safe passage of the baby during labor and delivery.
    • Depending on how much cervix is closed, placenta previa is divided into three types:
      • Complete previa: The placenta entirely covers the cervical opening.
      • Partial previa: The placenta covers some of the cervical aperture.
      • Marginal previa: It occurs when the placenta spreads to the cervix's border but does not cover it.
  2. Placenta accreta: The placenta attaches too deeply to the uterine wall but does not penetrate the uterine muscle. This is seen in 1 out of 2,500 pregnancies.
  3. Placenta increta: The placenta connects to the uterine wall further deeper and penetrates the uterine muscle. Placental increta accounts for roughly 15 percent of all cases.
  4. Placenta percreta: The placenta enters the uterine wall and joins to another organ such as the bladder. Placenta percreta is the rarest of the three disorders and accounts for just about five percent of all occurrences.

6 complications of the placenta

Complications related to the placenta are very rare, but when they occur, they can cause severe complications to the fetus and mother and include:

  1. Placenta previa: The placenta partially or completely covers the cervix, which is the entrance through which the baby will emerge. This problem is more frequent early in pregnancy and usually resolves as the placenta grows higher in the uterus. Cesarean delivery will be required if the placenta is still blocking the cervix near the time of delivery.
  2. Placenta accreta: The placenta develops too deeply into the uterine wall. This can result in severe blood loss during or after delivery, which can be fatal.
  3. Placental insufficiency: The failure of the placenta to give enough nutrition to the unborn fetus during pregnancy is referred to as placental insufficiency. This is caused by the placenta failing to grow or function properly, and it can lead to fetal growth limitation and low birth weight. Although there are no documented signs of placental insufficiency, the unborn baby may move less frequently than usual. During routine medical checkups, the healthcare provider can discover fetal growth restriction by measuring the height of the top of the uterus known as the fundus. An ultrasound scan can be used to monitor the status and size of the placenta, as well as the baby's health.
  4. Infarcts in the placenta: Within the placenta, there may be patches of dead tissue termed infarcts caused by diminished blood flow in specific locations. These infarcts are frequently caused by a problem with the placental vessels. Severe pregnancy-induced hypertension has been linked to an increase in the number of placental infarcts. In most cases, infarcts in the placenta do not affect the unborn child. However, in some circumstances, particularly in women with severe hypertension, the reduced blood flow in the placenta may be sufficient to cause poor growth and even death of the unborn baby.
  5. Placental abruption: Placental abruption is a pregnancy complication in which a portion or whole of the placenta separates from the uterus before the baby is delivered. The placenta has several blood vessels that transport nutrients from the mother to the unborn baby. When the placenta separates during pregnancy, these blood vessels rupture, resulting in bleeding. The more the bleeding, the larger the area that detaches.
  6. Retained placenta: If the placenta is not delivered within 30 minutes after the baby's delivery, it is said to be retained. It is a major issue because it can result in serious infection or life-threatening blood loss. Retained placenta is a rare but serious condition.

How does the placenta develop?

The chorion occurs about 10 days after conception as soon as the fertilized egg implants in the uterus. It is an embryonic organ that develops before the placenta.

The placenta is fully developed by week 18 to 20 of pregnancy, yet it continues to supply oxygen, nutrients, and immunity to the developing baby. By week 14 of pregnancy, the mother's blood supply is attached to the developing placenta.

The placenta is classified into two components:

  1. Maternal placenta: At roughly 7 to 12 days following conception, this component of the placenta develops from the mother's uterine tissue.
  2. Fetal placenta: When the blastocyst (the earliest form of the embryo) divides and burrows deep into the uterus to attach to the mother's blood supply, this component forms. It begins to form 17 to 22 days after conception.

The umbilical cord connects the placenta to the baby's abdomen, which is connected to the mother's uterus. The placenta is in charge of manufacturing pregnancy hormones and hosting vital nutritional exchanges between the mother's and baby's blood supplies.

The fetal blood is carried through the placenta, which is packed with maternal blood by small blood capillaries. Nutrients and oxygen are transferred from the mother's blood to the fetal blood, and waste is returned to the mother's blood—all without any mixing of the two blood sources.

What factors influence the conditions of the placenta?

Factors that influence the health of the placenta include:

  • Smoking history
  • High blood pressure
  • Multiple pregnancies
  • Maternal blood-clotting disorders
  • Uterine surgery, such as Cesarean delivery, in the past
  • History of placental issues
  • Cocaine usage by mothers (an example of maternal substance misuse)
  • Abdominal trauma such as that caused by a fall or physical assault
  • Maternal age (women older than 40 years are more likely to experience placental issues)
  • Preterm membrane rupture and premature amniotic sac rupture increases the risk of placental complications

What are the signs of placental complications?

If a pregnant woman develops the following symptoms, she must immediately seek medical attention because they may be caused by complications of the placenta and, if not treated in time, may lead to serious complications even death of the fetus.

Due to some specific complications of the placenta, vaginal delivery may not be possible or may cause serious complications. In such cases, the doctor performs a Cesarean delivery rather than normal vaginal delivery.

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Medically Reviewed on 1/27/2022
Image Source: iStock Images

Treating fever in adults: https://www.health.harvard.edu/staying-healthy/treating-fever-in-adults

What is the placenta? https://www.pregnancybirthbaby.org.au/what-is-the-placenta

Anatomy: Fetus in Utero: https://www.hopkinsmedicine.org/health/wellness-and-prevention/anatomy-fetus-in-utero

Bleeding in Pregnancy/Placenta Previa/Placental Abruption: https://www.chop.edu/conditions-diseases/bleeding-pregnancyplacenta-previaplacental-abruption