What Is the Placenta?

A Temporary Organ to Nourish the Fetus

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The placenta is an organ that develops in the uterus during pregnancy. It originates from the blastocyst shortly after implantation.

The role of the placenta is to provide oxygen and nutrients to the fetus and remove waste from the fetus’ blood. The placenta is attached to the uterine wall and connected to the fetus via the umbilical cord.

Complications associated with the placenta can be life-threatening. Because the organ functions to move blood and oxygen between the gestational parent and the fetus, the significant risk with placenta complications is hemorrhage.

3D conceptual artwork of a pregnant person showing a fetus and placenta

Sciepro / Getty Images

Function

The placenta functions to:

  • Transfer oxygen and carbon dioxide to and from the fetus
  • Transfer glucose, amino acids, fatty acids, electrolytes, vitamins, and water to the fetus
  • Produce necessary hormones for the fetus
  • Provide passive immunity to the fetus

The placenta has a maternal and a fetal side. The maternal side is attached to the endometrium, while the fetal side connects to the fetus via the umbilical cord.

The placenta used to be considered a barrier, protecting the fetus from outside contaminants. We now know that the placenta is only somewhat of a barrier.

Instead, most substances that the gestational parent absorbs or ingests can be transmitted through the placenta and umbilical cord to the fetus.

Variations

Usually, a placenta is a single organ that is circle- or oval-shaped. It usually attaches to the top, back (posterior), or front (anterior) side of the uterus. At term, the placenta weighs about a pound, is approximately 22 centimeters in diameter, and is 2 to 2.5 centimeters thick.

The placenta, however, can sometimes vary in shape, location, and size. Because of its critical function in moving blood and oxygen, variations can sometimes result in complications.

Usually, your healthcare professional will see the placenta during a prenatal ultrasound. If your placenta is in an unfavorable location or is shaped differently, your healthcare professional may choose to order further ultrasounds throughout your pregnancy to monitor your placenta.

Bilobed (or Multilobed) Placenta

A bilobed placenta is just as the name suggests—a placenta that separates into two nearly equal-sized lobes. Sometimes placentas will split into more than two lobes. Three lobes are called trilobes. Multilobe refers to a placenta that separates into more than one lobe. Bilobed placentas occur in 2% to 8% of pregnancies.

The most considerable risk associated with multilobed placentas is velamentous insertion of the cord. With velamentous insertion, the exposed umbilical cord veins and arteries insert into the amniotic membranes rather than the placenta.

Vasa previa (when unprotected umbilical cord veins lie over the cervical opening) occurs in 6% of cases of velamentous insertion. Vasa previa carries a risk of hemorrhage before delivery or at the time of delivery and can occur with rupture of membranes.

Succenturiate Placenta

A succenturiate placenta develops multiple lobes apart from the main body of the placenta. These lobes connect via the umbilical cord to the fetus. This variant occurs in 5% of pregnancies.

Those at greater risk for succenturiate placenta are those of advanced maternal age and those who have undergone in vitro fertilization (IVF). Complications include vasa previa and retained placenta.

Placenta Membranacea

Placenta membranacea is a rare abnormality where chorionic villi (tiny, finger-like projections of placental tissue) partially or wholly cover fetal membranes. The placenta is thin and covers the entire chorion (the outermost fetal membrane). Risks associated with this placental variation include placenta previa and placenta accreta.

Circumvallate Placenta

Circumvallate placentas are ring-shaped. In this variation, the fetal membranes fold back over the placenta.

These types of placentas are associated with poor outcomes due to the risk of premature rupture of the membranes, premature birth, placental abruption, small-for-gestational-age, neonatal intensive care unit (NICU) admission, and neonatal death.

Placenta Fenestrata 

With a placenta fenestrata, the central portion of the placenta is missing. This variation is rare. The most considerable risk associated with it is retained placenta at the time of birth.

Battledore Placenta

A battledore placenta is a variation in which the umbilical cord attaches at the edge of the placenta rather than the center. It occurs in 7% to 9% of singleton pregnancies and as frequently as 24% to 33% in twin pregnancies.

Complications associated with battledore placentas include preterm labor, fetal distress, and intrauterine growth restriction.

Complications

The placenta is a vital organ required to sustain the life of the fetus. When complications involving the placenta occur, they can be life-threatening both to the fetus and the parent.

Your healthcare provider will need to monitor complications closely. Complications of the placenta require carefully managed labor, often including a cesarean section.

Placenta Accreta

Placenta accreta is a complication where the placenta grows very deeply into the uterus. This fusion to the uterus makes it difficult for the placenta to separate from the uterine wall following delivery. This complication can lead to postpartum hemorrhage and is a life-threatening condition.

Placenta accreta occurs in 1 in 2,500 pregnancies. The rate increases with prior cesarean section, advanced maternal age, and placenta previa. There are no symptoms of placenta accreta before delivery. Fortunately, it can be suspected or diagnosed with routine ultrasound during pregnancy.

Planned cesarean is the safest treatment for placenta accreta. Blood transfusions are required as much as 95% of the time. Some cases may require a hysterectomy.

Increta and percreta are types of placenta accreta. Increta is when the placenta is embedded in the uterine wall and penetrates the myometrium.

Percreta is when the placenta develops through the uterine wall. It can even start growing into surrounding organs. These are the most life-threatening forms of placenta accreta.

Placenta Previa

When the placenta attaches low in the uterus and covers all or part of the cervical opening, it is known as placenta previa. It occurs in 1 in 200 to 250 pregnancies.

Risk factors include prior cesarean, prior dilation and curettage (D&C), smoking, carrying multiples, and advanced maternal age.

The main symptom of placenta previa is vaginal bleeding in the second or third trimester, but often, there are no prenatal symptoms. It is possible to see a low-lying placenta on a second-trimester ultrasound. When found, it should be closely monitored to see if it moves up and away from the cervix.

Placenta previa requires a cesarean delivery. If the placenta is low-lying but not covering the cervix, a vaginal birth may be an option.

Placental Insufficiency

Placental insufficiency, also known as placental deficiency, is a complication where the placenta fails to support a developing fetus properly. It can lead to growth restriction in the fetus and preeclampsia in the parent.

The main symptom of placental insufficiency is a smaller than expected abdomen, which indicates that the fetus is not growing as expected. People with high blood pressure, diabetes, and anemia, and those who smoke or use alcohol or drugs are at higher risk for developing placental insufficiency.

Premature birth, neonatal complications, neurological impairment, and stillbirth are possible outcomes of this complication.

If you notice any vaginal bleeding at any point during your pregnancy, be sure to tell your healthcare provider right away.

Risk Factors

Definitive causes of placental complications are unknown. But certain things put people at greater risk of developing placental complications.

You may be at greater risk of placental complications if you:

  • Smoke
  • Use drugs
  • Are over 35
  • Have been pregnant before
  • Are pregnant with multiples
  • Have had a prior cesarean, D&C, or other uterine surgery

A Word From Verywell

Most of the time placentas develop and function properly. Sometimes, however, complications can occur. Careful management of your birth will be required if you have any complications.

A C-section is often the best course of action when there are problems with the placenta. In severe cases of accreta, sometimes a hysterectomy is necessary. Hysterectomy can be a challenging diagnosis and treatment path, especially if you hope to have more children.

If you are diagnosed with placenta complications, talk to your healthcare professional or do research to find trusted online or in-person support groups to help arm you with information and reassurance.

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