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Fetal Structures and Functions

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Reference:

  1. Dr. RPS Maternal and Newborn Care: A Comprehensive Guide and Source Book for Teaching and Learning, 2nd Edition, ISBN 9789719822653, by Rosalinda Parado Salustiano (Ch. 4, 64–79)

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The Placenta

The placenta is a discoid (15 to 20 cm in diameter by 2 to 3 cm thick) product of conception in the uterus, positioned anteriorly or posteriorly near the fundus. Its base is always where the site of implantation occurs. It consists of two sides, the fetal side and the maternal side:

  1. Fetal Side: covered with amnion; under which fetal vessels course with the arteries passing over the veins. An amnion is 0.02 to 0.5 mm in thickness, a sac that engulfs the growing fetus. The clear fluid that accumulates within the amniotic cavity is the amniotic fluid.
  2. Maternal Side: irregular lobes consisting of fibrous tissue with sparse vessels confined mainly to the base.

By the time the pregnancy reaches term, the placenta is, on average, 500 grams in weight. The weight-ratio at term in an average fetus and placenta (feto-placental weight ratio) is 6:1 (3,000 grams to 500 grams).

The placenta forms with the union of the chorionic villi and the desidua basalis. The Decidua is the endometrium during pregnancy; thickening to 5 to 10 mm. It consists of multiple layers and sublayers:

  1. Decidua Basalis: the base layer, under the embedded ovum, directly under the site of implantation.
  2. Decidua Capsularis: the portion overlying the developing ovum; separates the ovum from the rest of the uterine cavity; most prominent by the second month.
  3. Decidua Vera/Desidua Perietalis: lines the remainder of the uterus.
  4. Layers of the Decidua Basalis and Vera:
    • Zona Compacta: uppermost/surface layer made up on compact cells.
    • Zona Spongiosum: the middle, spongy layer; with glands and small blood vessels. Along with zona basalis, it is the functional layer (zona functionales) as it is to this layer that implantation occurs.
    • Zona Basalis: lowermost/basal layer. This is regenerated into a new endometrium after placenta separation.
  5. Decidual Aging: Nitabuch’s Layer, a zone of fibrinoid degeneration, is where the invading trophoblast meets the decidua. This layer is usually absent whenever the decidua is defective.

Placental Maturity

The placenta begins developing within the second month of pregnancy, and becomes completely functional on the third month of gestation. It functions most effectively through 40 to 41 weeks, and may be dysfunctional beyond 42 weeks.

Placental functions vary, and play vital roles in nutritive, respiratory, excretory, protective, and endocrine systems:

  1. Nutritive: transports nutrients and water-soluble vitamins to the fetus via fluid/gas transport.
  2. Respiratory: serves as the fetal organ of respiration.
  3. Excretory: the amniotic fluid is the medium of excretion.
  4. Protective: the placental barrier defends against organisms and substances like heparin and bacteria. It is ineffective against viruses, alcohol, nicotine, antibiotics, depressants, and stimulants.
  5. Endocrine: production of estrogen, progesterone, human chorionic gonadotropins (hCG), and human placental lactogen (hPL), also called chorionic somatomammotropin (hCS).
    • The major source of estrogen and progesterone after the first two months.
    • hCG: secreted as early as 8 to 10 days after fertilization, detected in serum as early as the time of implantation by the most sensitive pregnancy test, radioimmunoassay (RIA). Detected in urine by 10 days (2 weeks) after the first amenorrheic period. It prolongs the lifespan of the corpus luteum, and serves as the basis of pregnancy tests. It is found to be elevated in cases of excessive vomiting. Normally, its value is 50,000 to 400,000 IU/24 hours.
    • hCS/hPL: secreted by the third week after ovulation, influencing somatic cellular growth (resembling growth hormone). It is the principal diabetogenic factor, being a major insulin antagonist/glucose sparing hormone. It prepares the breasts of the mother for lactation.

Placental/Cord Abnormalities

  1. Placenta Succenturiata
  2. Placental Infarcts
  3. Placenta bipartita
  4. Placenta tripartita
  5. Placenta circumvallata
  6. Battledore placenta
  7. Velamentous insertion of the cord
  8. Cord loops
  9. Cord torsion
  10. Cord knots

The Umbilical Cord

Also known as the funis, it is a 30 to 100 cm (average 55 cm) cord with a diameter of 0.8 to 2.0 cm at term. It extends from the fetal surface to the placenta to the fetal umbilicus, serving to transport oxygen and nutrients, and returning metabolic wastes from the fetus to the placenta. It contains three umbilical vessels (mn. AVA): one vein and two arteries. These cords spiral in a dextral or sinistral manner, believed to prevent clamping.

  1. Umbilical Vein: carries oxygenated blood to the fetus.
  2. Umbilical arteries: carries deoxygenated blood from the fetus to the placenta.

Protective Covering of the Funis

The cord is covered by Wharton’s Jelly, a specialized gelatinous connective tissue, an extension of the amnion that prevents cord compression.

The Amniotic Fluid

A clear, straw-colored fluid in which the fetus floats. It appears from both the fetal and maternal placenta, amniotic epithelium, maternal serum, and in the latter part (10th week), fetal urine. It is continuously replaced to prevent “dry labor” in a premature rupture of the bag of water. It reaches 500 to 1,000 mL. More than this results in polyhydramnios (1,000 to 1,500 mL), and less results in oligohydramnios (300 to 500 mL). Its pH is normally neutral to alkaline (pH 7.00 to 7.25).

It functions as a protective cushion/shock absorber. It separates the fetus from membranes, allowing symmetrical growth and free movement. As previously mentioned, it is the medium by which wastes are excreted from the fetus. It also serves as a fetal drink.

  • The amniotic fluid can be used as a specimen for periodic diagnostic exams (amniocentesis) to determine fetal well-being or its absence.
  • Fetal temperature is also maintained by amniotic fluid.
  • The liquid prevents marked interference with placental circulation during labor.

Abnormalities

  1. Polyhydramnios: an elevation in amniotic fluid volume, over 1,000 mL.
    • One of the causes of polyhydramnios is when the fetus is unable to drink; if an abnormality in the deglutition center of the brain or if there is esophageal atresia that the fetus cannot swallow.
  2. Oligohydramnios: a decrease in amniotic fluid volume, under 500 mL.
  3. Meconium-stained amniotic fluid, especially if the fetus is non-breech, is a sign of fetal distress.
  4. Golden-colored amniotic fluid may be found in hemolytic disease.

The Embryonic Germ Layers

The three main stages of the embryonic period are gastrulation (the ectoderm, mesoderm, and endoderm form), neurulation (formation of neural tissue), and organogenesis (development of organs).

The three embryonic germ layers or layers of the blastocyst:

  1. The Ectoderm is the outer layer, develops into the following:
    • The nervous system
    • Hair, nails, skin epidermis, sebaceous and sweat glands
    • Salivary glands, and mucous membranes of the mouth
    • Epithelium of nasal and oral passages
  2. The Mesoderm is the middle layer, develops into the following:
    • Dermis
    • Cardiovascular system
    • Reproductive system
    • Musculo-skeletal system
    • Urogenital system, except the bladder
  3. The Endoderm/Entoderm is the inner layer, develops into the following:
    • Linings of the gastrointestinal tract from the pharynx to the rectum
    • Liver, pancreas, thyroid, and parathyroid
    • Respiratory tract
    • Bladder, and thymus (for immunity building)

Memory Tool

There are general divisions of “purpose” that may be attributed to each layer. While not all-encompassing, they can do the job for many of the parts in each layer.

  1. The ectoderm includes structures that the person may use to “interact with the world”—the nervous system, the integument and epidermis for touch, the mouth and its salivary glands for food, and epithelium of the nasal and oral passages for smell.
  2. The mesoderm is for “movement, muscles, middle”. Also relate “movement” to “flow”. It includes the muscles and skeleton that move the body and cardiovascular system that moves blood (flow). “Middle” refers to the dermis, the middle layer of the skin. “Flow” can be used to recall the reproductive and urogenital components.
  3. The endoderm is for “inside” (as the name suggests). It’s where the inner linings (lining of gastrointestinal and respiratory tracts) arise, and various glands (liver, pancreas, thyroid, parathyroid). Tack on the thymus and bladder.

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