The placenta, umbilical cord, and amniotic sac

Last updated: April 27, 2022

Summarytoggle arrow icon

The placenta, umbilical cord, and amniotic sac protect and provide nutrients to the fetus. The placenta is a fetomaternal organ that enables the selective transfer of nutrients and gases between mother and fetus. The placental barrier limits direct contact between the embryo and maternal blood, thus protecting both mother and child from potentially harmful substances (e.g., blood cell antigens of the unborn, bacteria from the mother). In addition, the placenta produces hormones that mediate maternal adaptation to pregnancy and maintain pregnancy. Establishing uteroplacental circulation involves several steps, including endovascular trophoblast invasion and uterine vascular remodeling. The 50–70 centimeter long umbilical cord connects the placenta with the fetus and contains one umbilical vein that carries oxygenated, nutrient-rich blood supply and two umbilical arteries that carry deoxygenated blood from fetus to the placenta and the maternal circulation. The amniotic sac surrounds the fetus and contains the amniotic fluid, providing mechanical protection to the developing fetus.

Development of uteroplacental circulationtoggle arrow icon

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

Decidual reaction


Placentation refers to the development of the placenta. The embryonic portion of the placenta is derived from cells of the trophoblast and the maternal portion of the decidua basalis.

The placentatoggle arrow icon

Placental structure

At term, 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 (placenta)

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 barrier consists of six 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 functiontoggle arrow icon

Overview of placental hormones

The most important placental hormones are HCG, HPL, CRH, estrogen, and progesterone; other important hormones during pregnancy include thyroid hormones, oxytocin, and prolactin.

Hormones and the placenta

Hormone Site of production Effect(s) Course during pregnancy
Placental hormones
hCG (human chorionic gonadotropin)
  • Rapid increase of β-HCG during early pregnancy
    • Concentration doubles every 29 to 53 hours during the first 30 days after implantation
    • Maximum concentration: ∼ 10th week of gestation (week 8 of embryonic development)
hPL (human placental lactogen)
CRH (corticotropin-releasing hormone)
  • 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
  • Rapid fall after delivery

Other hormones

Thyroid hormones
  • Vital for fetal neurologic development (e.g., myelination in the CNS)
  • Induces uterine contractions
  • Facilitates milk ejection reflex
  • Considered to play a key role in bonding between mother and child, social interactions, and maternal mood (e.g., postpartum “blues”, postpartum depression)
  • Contraction of uterine muscle after delivery of the placenta
  • Increases throughout gravidity, peaks during late pregnancy and labor
  • Elevated plasma levels until ∼ 6 months postpartum

Gas and nutrient exchange

The placenta is the main site of metabolites and gas exchange between the mother and the fetus.

Fat-soluble vitamins (A, D, E, K), immunoglobulins (except IgG), and most proteins are either unable to cross the placental barrier or have 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 cannot cross the placental barrier!

The umbilical cordtoggle arrow icon

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. The development of the umbilical cord begins at approx. the 3rd week of embryonic development. By the end of pregnancy, the umbilical cord is approx. 50–70 cm long.

Formation and structure of the umbilical cord

Structure and development of the umbilical cord during early pregnancy

Structure of the umbilical cord during late pregnancy

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

A single umbilical artery is a sign of chromosomal disease and congenital anomalies. [2]

Physiological umbilical hernia

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

Amnion and amniotic cavitytoggle arrow icon

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, eventually resulting in the displacement of the chorionic cavity and the uterine cavity.

Amniotic sac

The amniotic sac is composed of maternal (decidua) and fetal components (chorioamniotic membranes) that 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.

Referencestoggle arrow icon

  1. Burton GJ, Hempstock J, Jauniaux E. Nutrition of the Human Fetus during the First Trimester—A Review. Placenta. 2001; 22: p.S70-S77.doi: 10.1053/plac.2001.0639 . | Open in Read by QxMD
  2. Murphy-Kaulbeck L, Dodds L, Joseph KS, Van den Hof M. Single Umbilical Artery Risk Factors and Pregnancy Outcomes. Obstetrics & Gynecology. 2010; 116 (4): p.843-850.doi: 10.1097/aog.0b013e3181f0bc08 . | Open in Read by QxMD

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 Evidence-based content, created and peer-reviewed by physicians. Read the disclaimer