The Placenta: Characteristics and Functions

The placenta constitutes a close link between the mother and the fetus through which it obtains oxygen and nutrients.
The Placenta: Characteristics and Functions

Written by Fabiola Marín

Last update: 12 April, 2023

The placenta is an organ that develops in the uterus during pregnancy and connects the maternal blood with that of the fetus. It allows the gas and nutrient exchange and the development of metabolic, endocrine, and immunological activities.

Formation of the placenta

The formation of the placenta takes place when the fertilized egg implants itself in the uterine wall. It grows with the fetus until shortly before delivery, which doesn’t end until it’s completely expelled.

The mature placenta can measure between 15 and 25 cm (6 to 10 inches) in length, and 3 cm (just over 1 inch) thick. It usually weighs approximately 500 or 600 grams (17 to 21 ounces), with the ratio between placental weight and fetal weight being 1/5.

The outer part of the placenta comes from the changes that occur in the lining of the uterus. The internal part is called trophobast and is formed from blasts from the embryo itself.

From the trophoblast, a network of blood vessels is established through which the fetus can receive nutrients, and chorionic villi are formed, which allow the embryo to attach to the uterine wall.

The placenta.

What’s it for?

The main functions of the placenta are:

  • Transmitting nutrients to the fetus
  • Excreting waste substances into the mother’s blood
  • Hormonal production, which allows the maternal metabolism to be modified in the different stages of pregnancy.

It’s a fairly complex organ involved in a multitude of ‘vital’ functions, which we can classify into the following:

Transportation and exchange of substances

Through the placenta, the exchange of respiratory gases and nutrients takes place between the maternal and fetal vascular systems. It acts as a barrier to the metabolites, hormones, and toxins found in the maternal circulation.

Endocrine function

In the placenta, the synthesis of growth factors and hormones essential for the correct development of the fetus and the mother in the evolution of pregnancy takes place:

  • Chorionic gonadotropin hormone (hCG): It’s essential in the evolution of pregnancy.
  • Placental Lactogen (hLP): Promotes fetal anabolism, increases insulin secretion, improves
    glucose tolerance, and favors nitrogen fixation in fetal tissues.
  • Progesterone: Prepares the endometrium for the implantation of the embryo in the uterine wall.
  • Estrogens: These facilitate the adaptation of the maternal cardiovascular system to pregnancy and the development of the mammary gland.

Immune function

This is essential so that the mother’s immune system doesn’t identify the fetus as a foreign body and react to it. It also acts as a barrier against different pathogens.

Possible complications

Retained placenta

At the time of birth, if the placenta hasn’t detached in 30 minutes, it’s considered a retained placenta and it’s necessary to proceed to perform maneuvers, such as:

  • Uterine massage and bladder catheterization
  • Administration of oxytocin
  • Credé maneuver

If this isn’t enough, then manual removal of the placenta is performed.

Previous placenta

It occurs when the placenta fully or partially inserts into the cervix. It’s classified into 4 types depending on the location, during pregnancy:

  • Type I: lateral or low placenta. The placental border is implanted in the lower uterine segment.
  • Type II: marginal placenta. It reaches just to the edge of the internal cervical os, but doesn’t exceed it.
  • Type III: partial occlusive placenta. Partially covers the internal cervical os.
  • Type IV: total occlusive placenta. Fully covers the internal cervical os.

Regarding delivery, it’s classified into:

  • Occlusive: Doesn’t allow vaginal delivery.
  • Non-occlusive: Allows attempted vaginal delivery.

The most characteristic symptom associated with placenta previa is genital hemorrhage, which has to the following characteristics:

  • Red, bright blood of variable quantity.
  • It appears abruptly and painlessly and in the absence of uterine contractions.
  • It usually appears in the second or third trimester of pregnancy.

The position of the placenta isn’t modifiable, so there’s currently no treatment for this complication. Only in the case of severe bleeding, an emergency cesarean section will be performed.


In conclusion, the placenta is an extraordinary organ that is formed at the moment the ovum is fertilized and implanted in the wall of the uterus. Later, it’s expelled after delivery.

It represents a close link between mother and fetus through which it obtains oxygen and nutrients. It’s involved in important endocrine functions that will favor the evolution of pregnancy. It also acts as a protective barrier against foreign external agents.

  • Iglesias Guiu J, Martín Jiménez A (2007). Fundamentos de obstetricia (SEGO). Morfogénesis y morfología de la placenta a lo largo de la gestación. Fisiología de la unidad feto-placentaria.
  • Iglesias Guiu J, Martín Jiménez A (2007). Fundamentos de obstetricia (SEGO). Placenta previa: concepto y clasificación.
  • Burton GJ, Jauniaux E (2015). What is the placenta?. American journal of obstetrics and gynecology, 213(4 Suppl):S6.e1, S6-8.

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