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.
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: (decidualization): implantation → thickening and structural changes of the endometrium → formation of decidua
Function of decidua
- Histiotrophic nutrition
- Immune privilege: tight junctions separate the conceptus from adjacent endometrial tissue
- Preparation for placental circulation: increased progesterone → transformation of decidual vessels into a network of anastomosing spiral arteries (uterine vascular remodeling)
- The decidua has three distinct parts, distinguished by their relation to the site of implantation:
Early placental development
- Prelacunar stage: until approx. day 9 of embryonic development
- From approx. day 9 of embryogenesis
- Lacunae form in syncytiotrophoblast, these are separated by thin syncytiotrophoblast trabeculae
- Lytic enzymes of syncytiotrophoblast eventually erode the spiral arteries of the decidua, and maternal blood fills the lacunae.
- Lacunae merge to form the intervillous space.
- Hemotrophic nutrition: nutrient supply from maternal blood 
- Early villous stage: from approx. day 13–28 of embryogenesis
Chorionic villi development and maturation: the composition of chorionic villi changes during the course of placental development.
- Primary villi
- Secondary villi
Tertiary villi (terminal villi): connect to the umbilical cord vessels during week 3 of embryonic development
- Tertiary villi develop through vascularization
- Terminal villi develop after the 4th month → cytotrophoblast cells begin to disappear → with only isolated cytotrophoblast cells (Langhans cells ) remaining
- Structure of terminal villi
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 (decidua basalis): mainly maternal component
- Intervillous space and villous trees: fetomaternal zone
- Chorionic plate: fetal component
Basal plate (placenta)
- Description: mainly maternal component, abuts the uterine wall
- Structure: maternal decidua with invading embryonal cells (cytotrophoblast, syncytiotrophoblast, and extravillous trophoblast cells)
- Blood supply: : uterine spiral arteries
- Intervillous space
Villous trees: The placenta is composed of 30–50 branching villous trees.
- Stem villi: basal region of villous trees with fetal arteries and veins
- Intermediate villi: region of villous trees with fetal arterioles, venules, and capillaries; important region of gas and nutrient exchange
- Terminal villi: tertiary villi that float freely in the intervillous space and are directly involved in gas and nutrient exchange
- Anchoring villi: anchor the villous trees to the decidua
- Description: : Fetal component of the placenta
- Structure: formed by the syncytiotrophoblast, the cytotrophoblast, and the somatic layer of the extraembryonic mesoderm
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).
- Structure until the 4th month of embryonic development (from maternal to fetal)
- Structure from the 4th month of embryonic development (from maternal to fetal)
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.
- Site of production: syncytiotrophoblast
- Function of hormones
|Hormone||Site of production||Effect(s)||Course during pregnancy|
|hCG (human chorionic gonadotropin)|
|hPL (human placental lactogen)|
|CRH (corticotropin-releasing hormone)|| |
|Progesterone|| || |
|Thyroid hormones|| |
Gas and nutrient exchange
- Passive transport
- Active transport: amino acids, peptides, hormones, vitamins, fatty acids, inorganic ions
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 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
- The umbilical cord contains 3 allantois-derived blood vessels that carry fetal blood:
Structure and development of the umbilical cord during early pregnancy
- Connecting stalk: precursor of the umbilical cord
- Connects the midgut to the yolk sac
- Obliterates during 6–7th week
- Failure to fully obliterate can lead to the following conditions
Structure of the umbilical cord during late pregnancy
- ECM: Wharton jelly (gelatinous connective tissue)
- Cover: amniotic epithelium
- Urachus (duct between fetal bladder and umbilicus)
- Remnants of the obliterated vitelline duct
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 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.
- Development: 2nd week of development through migration of epiblast cells
- Outermost membrane
- Develops from the decidua capsularis, which lies above the site of implantation
Composition: initially a clear liquid
- Amount: approx. 850–1500 mL by the end of pregnancy (the amniotic fluid is completely exchanged every 3 hours)
- pH: 7–7.5 (slightly alkaline)
- Proteins, glucose, urea
- Fetal urine, hair, dead skin, sebum
- Vernix: a milky-white, lipid-rich substance that consists of fetal dermal cells and sebaceous gland secretions. It covers the fetus's skin (especially in the third trimester).