The placenta, umbilical cord, and amniotic sac

Development of uteroplacental circulation

Following implantation, the endometrial stromal cell lining is transformed into the decidua (decidual reaction). The decidua provides nourishment to the embryo until the definitive placenta forms. Approximately on day 12 of embryonic development, fetal blood vessels (through opening of the maternal vessels) come into contact with maternal blood, forming a region of fetal-maternal exchange.

Decidual reaction



The placenta

Placental structure

At the end of pregnancy, the mature placenta weighs approx. 500 g, is about 2 cm thick, and has a diameter of 15–20 cm. It consists of three parts:

Basal plate (decidual basalis)

Intervillous space and villous trees

Chorionic plate

Placental barrier

Maternal and fetal circulation are separated by the placental barrier. The placental barrier controls the gas and nutrient exchange. Until the fourth month of development, the placental barriers consists of five layers. After the fourth month, the cytotrophoblast disappears from the villous wall, leaving only the isolated cytotrophoblast cells (Langhans cells).

After birth, the placenta must be inspected to ensure it has detached completely from the uterine wall. If this does not occur, there is a risk of postpartum hemorrhage. The check is performed by inspecting for the completeness of all placental cotyledons. On the fetal side, the placenta should be covered by the amnion!

Placental function

Hormone production

Hormone Site of production Effect(s) Course during pregnancy
hCG (human chorionic gonadotropin)
hPL (human placental lactogen)

CRH (corticotropin-releasing hormone)

  • Is thought to play a role in determining the duration of gestation, in maturation of the fetal lung, and in surfactant production
  • Increases during pregnancy or maternal stress
  • Increases continuously until birth
  • Optimizes transport of zygote along the fallopian tube and subsequent implantation

  • Maintains pregnancy

  • Transformation of endometrium (proliferative function → secretory function)

  • Prevents menstruation (degradation of endometrium)

  • Closure of the cervix and increased viscosity of cervical secretion

  • Inhibits uterine contractions

  • Stimulates the expression of uterine oxytocin receptors prior to delivery

  • Inhibits further differentiation of breast tissue in the second half of pregnancy

  • Following delivery, rapid decline of progesterone triggers the secretory activation lactogenesis phase of lactogenesis marked by copious milk production

  • Increases continuously until birth
Thyroid hormones
  • Vital for fetal neurologic development (e.g., myelination in the CNS)
  • Induces uterine contractions
  • Promotes bonding between mother and child

Gas and nutrient exchange

Fat-soluble vitamins (A, D, E, K), immunoglobulins (except IgG), and most proteins are either unable to cross the placental barrier or have an only limited ability to do so. Vitamin K is an important cofactor for blood coagulation and should be administered to the newborn infant directly after birth!

Anti-D antibodies from the Rhesus system (= IgG antibodies) are able to cross the placental barrier. In contrast, isoagglutinins of the ABO system are mainly IgM antibodies, which means that they are unable to cross the placental barrier!

The umbilical cord

The umbilical cord connects the fetus with the fetal part of the placenta (chorionic plate). It typically attaches centrally to the chorionic plate of the placenta. Development of the umbilical cord begins at approx. the 3rd week of embryogenesis. By the end of pregnancy, the umbilical cord is approx. 50–70 cm long.

Formation and structure of the umbilical cord

Early stage in the development of the umbilical cord

Late stage in the development of the umbilical cord

The umbilical arteries carry deoxygenated blood, whereas the umbilical vein carries oxygenated blood!

Physiological umbilical hernia

Due to the rapid growth of the gastrointestinal tract, there is a short period of time during which there is not enough space within the embryonic abdominal cavity. As a result, sections of the gut herniate into the extraembryonic coelom of the future umbilical cord from the 6th–10th week of development.


Amnion and amniotic cavity

Amniotic cavity

The amniotic sac is formed very early in pregnancy and surrounds the embryo as a protective shell. As the fetus grows, the amniotic cavity expands, which eventually results in the obliteration of the chorionic cavity and the uterine cavity.

Amniotic sac

The amniotic sac is composed of maternal (decidua) and fetal components (chorioamniotic membranes) which surround the fetus and provide mechanical protection.

Amniotic fluid

Protective fluid within the amniotic sac that cushions the fetus, prevents adherence of the fetus to the amnion, and serves as a transport medium for nutrients and metabolites.

Clinical significance

  • 1. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 2. Pijnenborg R. Establishment of uteroplacental circulation. Reprod Nutr Dev. 1988; 28(6B): pp. 1581–6. pmid: 3247514.
  • 3. Degner K, Magness RR, Shah DM. Establishment of the human uteroplacental circulation: A historical perspective. Reprod Sci. 2016; 24(5): pp. 753–761. doi: 10.1177/1933719116669056.
  • 4. Rodeck CH, Whittle MJ. Fetal Medicine: Basic Science and Clinical Practice. Elsevier Health Sciences; 2009.
  • 5. Hill M. Placenta - Villi Development. Updated August 21, 2018. Accessed November 18, 2018.
  • 6. Baergen RN. Manual of Benirschke and Kaufmann’s - Pathology of the Human Placenta. Springer-Verlag; 2018.
  • 7. Le T, Bhushan V, Sochat M, Chavda Y. First Aid for the USMLE Step 1 2017. McGraw-Hill Education; 2017.
  • Roberts V, Myatt L. Placental Development and Physiology. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated August 29, 2018. Accessed November 18, 2018.
  • Murphy-Kaulbeck L, Dodds L, Joseph KS, Van den Hof M. Single Umbilical Artery Risk Factors and Pregnancy Outcomes. Obstetrics & Gynecology. 2010; 116(4): pp. 843–850. doi: 10.1097/aog.0b013e3181f0bc08.
last updated 10/17/2020
{{uncollapseSections(['h6cckW0', '36cSkW0', 'R6clkW0', 'i6cJkW0', 'Q6cukW0', '7Cc4te0'])}}