Human Placenta: Description, Functions and Complications

Appearance of the placenta at term

Human placenta is a circular flat organ through which the fetus obtains oxygen and nourishment in-utero and performs functions which the fetus cannot perform itself in-utero. Its name is derived from Latin for “pancake,” which is descriptive of its size and appearance at term. It is also describe as a descoid-shaped organ, which otherwise called “afterbirth“.

Situation/Location: Before delivery, the placenta is situated in the upper uterine segment.

Shape: The human placenta is a flat, roughly circular structure.

Size: It’s about 20–22cm in diameter and 2.5cm thick in the centre and becomes thinner towards the circumference.

Weight: Normal human placenta weighs about one-sixth of the baby’s weight(450g).

Gross structure of Human Placenta

The human placenta has two surfaces which are: maternal and fetal surfaces.

Maternal surface

This surface is dark red in colour due to maternal blood in intervillous spaces and part of the basal decidua will have been separated with it.

The surface is arranged in about 18–20 lobes which are separated by sulci.  The lobes are made of lobules, each of which contains a single villus with its branches. Sometimes, deposits of lime salts are present — making the surface gritty. Although it has no clinical significance.

Fetal surface

This is the surface that faces the baby in-utero and it has a white shiny appearance or bluish gray in colour with a smooth surface.

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The amnion covering the fetal surface of  the  placenta  gives  it  a whitish, shiny  appearance. 

Branches of the umbilical veins and arteries are visible and spreading out from the insertion of the umbilical cord which is normally in the centre..

The umbilical cord is inserted usually at the centre and blood vessels radiating down to its circumference.

The amniotic membrane covers the fetal surface, which consists of a double membrane: 

The Chorion: This is derived from the trophoblastic tissue. It is a thick, opaque, friable membrane which continues with a placental edge and cannot be separated from it. 

The chorion is the outer layer of amniotic membrane adherent to the uterine wall.

The Amnion: This is a smooth, tough translucent membrane derived from the inner cell mass. It covers the placenta and the umbilical cord and secretes the amniotic fluid.

The inner layer of the amniotic sac contains an amniotic fluid and covers the fetal surface of the placenta and is what gives the placenta its typical shiny appearance.

Human Placenta

Functions of human placenta

Respiratory function: As pulmonary exchange of gases does not take place in the uterus the fetus must obtain oxygen and excrete carbon dioxide through the placenta. It also aids exchange of oxygen and carbon dioxide from the mother to the fetus.

Nutrition functions: Food for the fetus derives from the mother’s diet and has already been broken down into forms by the time it reaches the placental site. Nutrients such as glucose, amino acids, and minerals pass into the foetal circulation through it. The placenta is able to select those substances required by the fetus, even depleting the mother’s own supply in some instances.

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Storage: It metabolizes glucose and can also store it in the form of glycogen and reconverts it to glucose when it is needed by the fetus. It also stores  iron  and  other  fat  soluble vitamins.

Excretory functions: The waste products of metabolism such as urea, or  uric acid are returned to maternal circulation through the placenta. The main substance excreted from the fetus is carbon dioxide. 

Protective functions: It provides partial protection from infection (e.g. micro-organisms). It provides a limited barrier to infection with the exception of the treponeona of syphilis and few bacteria can penetrate. Viruses, however, can cross freely and may cause congenital abnormalities as in the case of the rubella virus and HIV virus.

Endocrine functions: It produces hormones such as Human   chorionic   gonadotropin (HCG) produced by the cytotrophoblastic layer of the chorionic villi; human placental lactogen(hpL) that has a role in glucose metabolism in pregnancy; oestrogen and progesterone which all help to maintain the viability of the gestation. 

When  the activity of the corpus luteum begins to decline, the  placenta  takes  over  the  production  of  oestrogen, which   is   secreted   in   large   amounts   throughout pregnancy.

Anchorage/stability: The chorionic villi passes deeply into the decidua and anchors the placenta firmly.

Examination of placenta

Requirements for the procedure

  • Two clean gloves, one plastin apron
  • Placenta in a bowl of water
  • Metal probe

Aims of the examination

  • To ensure conduct of labour is complete
  • To detect abnormality
  • To ensure its completeness.

Procedure

  • Hold it by the cord.
  • Check the blood vessels using the probe, which should be two umbilical arteries and one vein in a normal condition.
  • Lift it up and let it hang and then note the length of the cord.
  • Note the weight.
  • Check the true/false knot and the size of the cord, and the amount of Wharton’s jelly.
  • Note cord insertion, which should be central in a normal placenta.
  • Check the opening on the membranes whether smooth or ragged. If ragged, then suspect some retained membranes.
  • Separate the chorion from the amnion.
  • Note presence of any abnormality; for instance, placenta circumvallate or velamentosa.
  • Turn the maternal surface and cup it —removing retroplacental clots, infarcts or identify missing lobes.
  • All abnormalities detected on examination must be recorded and reported.
  • The placenta is then put into the covered bowl for disposal.
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Complications Arising from Human Placenta

Placenta remains an essential organ that connects mother and foetus for exchange of gases and nutrients as well as removal of waste products of metabolism. However, anomalies or complications can occur in it which include: 

  • Abnormalities which affect fetal growth and development, and can cause intrauterine growth restriction or intrauterine fetal death
  • Retained placenta resulting in postpartum haemorrhage
  • Placenta praevia — risk factor of postpartum haemorrhage
  • Placental abruption —which is also associated with  antepartum haemorrhage