Module 9.0

DMST 244 Obstetrics & Gynecology I - Module 9: 2nd and 3rd Trimester Notes

Fetal Environment

• Key Topics: Placenta and Cord

Questions to Consider

1. Does the maternal blood and fetal blood mix during pregnancy?

2. Does the placenta protect the fetus?

3. Can a fetus survive with a 2 vessel cord?

4. Can we assess the function of the placenta by imaging it?

5. How is placental function assessed?

6. What purpose is placental grading and when is it performed?

Topics

• Anatomy of the Placenta

• Function of the Placenta

• Sonographic Appearance
    - Size and Shape of the Placenta

• A Touch of Pathology

Placenta

• Definition: The placenta is classified as an organ with multiple functions:
    - Provides nutrition to the fetus
    - Transfers oxygen to the fetus
    - Removes carbon dioxide from the fetus
    - Eliminates fetal waste products (e.g., urea, uric acid, bilirubin)
    - Regulates fetal temperature
    - Excretes hormones like hCG (human chorionic gonadotropin)

Anatomy - Components

• Distinct Components by 12 Weeks Gestation:
    - Fetal Portion: Chorion frondosum
    - Maternal Portion: Decidua basalis

• Connection: These portions are held together by anchoring or stem villi at the cytotrophoblastic shell.

Cotyledons

• Definition: Each compartment of the placenta is called a cotyledon and is considered the functional unit of the placenta.

• Components of a cotyledon:
    - Composed of several stem villi.

• Approximate number of cotyledons in a placenta: 20

Placental Anatomy

• Developments:
    - Syncytiotrophoblast cells create large intervillous spaces (lacunae) in the decidua basalis.
    - Projections called placental septa divide the placenta into compartments known as cotyledons.

• Placental Membrane: Separates fetal blood in capillaries of chorionic villi from maternal blood in intervillous spaces.
    - Maternal blood enters via spiral arterioles.

Circulation

• Maternal Blood Flow: The spiral arterioles shoot blood around the chorionic villi.

• Fetal Blood Flow: It moves from umbilical arteries to chorionic villi, then back to the umbilical vein to the fetus.

Functions of the Placenta

1. Respiration:
      - Transfer of oxygen, carbon monoxide, and carbon dioxide across the placental membrane, acting as the fetus's lungs.

2. Nutrition:
      - Synthesis and transfer of water, inorganic salts, carbohydrates, fats, and proteins from maternal blood to fetal blood.

3. Excretion:
      - Waste products in fetal blood cross to maternal blood for excretion by the mother's kidneys.

4. Protection:
      - Most microorganisms do not cross the placental barrier; however, certain agents (teratogens) including drugs and some viruses can affect fetal development.

5. Storage:
      - Nutrients are stored in the placenta and released into fetal circulation as needed (e.g., carbohydrates, proteins, calcium, iron).

6. Hormonal Production: Made primarily by the syncytiotrophoblast of the placenta,
      - Includes hormones such as human chorionic gonadotropin (hCG), estrogens, progesterone, and human chorionic somatomammotropin (hPL).

Sonographic Appearance of the Placenta

• Early Appearance:
    - Diffuse granular echo pattern observed as early as 10-12 weeks.

• Texture Changes:
    - Basic texture remains stable, with an increase in calcium deposits over time.
    - Placental calcifications appear in over 50% of placentas after 33 weeks gestation.
    - Increased calcification is observable in cases of IUGR (Intrauterine Growth Restriction), hypertension, and smoking.

Grading the Placenta

• Grade 0: Homogeneous appearance with a straight chorionic plate.

• Grade I: Scattered echogenic areas and subtle undulations.

• Grade II: Presence of indentations and linear echogenic areas (comma-like densities).

• Grade III: Significant indentations, cystic areas, and shadowing calcifications (after 36 weeks).

Size and Shape of the Placenta

• General Characteristics:
    - Flat and circular; typical weight ranges from 500 to 600 grams; thickness ranges from 1.5 cm to 4 cm.
    - A normal placenta grows approximately 1 mm per week in thickness.
    - A thickness over 4 cm is considered abnormal.

Pathological Conditions

• Placenta Megaly: Thickness greater than 4 cm linked to several conditions such as maternal diabetes, placental hemorrhage, and chromosomal abnormalities.

• Thin Placenta: Less than 1.5 cm may result from vascular deficiencies, pre-eclampsia, and may lead to IUGR.

Umbilical Cord Anatomy

• Structure: Develops from the connecting stalk and yolk sac; typically consists of 2 arteries and 1 vein surrounded by Wharton’s Jelly and covered with amnion; length varies from 50 to 100 cm.

• Attachments:
    - Cord attaches centrally to the placenta and travels to the fetus where arteries lead to the hypogastric arteries and the vein progresses toward the portal sinus.

• Single Umbilical Artery:
    - Presence of only 2 vessels may correlate with cardiac or renal anomalies and often requires further evaluation via fetal echo.
  
  

Other Cord Anomalies

• Examples include:
    - Cord Cyst: Usually insignificant.
    - Hematoma: Associated with trauma or wall weakness, has a high risk of perinatal death.
    - Umbilical Vein Thrombus: Can cause severe complications including perinatal death.
    - Umbilical Vein Varix: Involves focal enlargement of the umbilical vein, can lead to stasis and increased risk of adverse outcomes.

Doppler Analysis in Ultrasonography

• Utilized for examining blood flow within the umbilical cord; helps in assessing vessel patency and abnormalities.

Learning and Practical Implications

• Understanding placental grading, function, and pathologies is crucial for monitoring fetal health and making informed clinical decisions during pregnancies.

References

• Raatz Stephenson, S. (2012). Diagnostic Medical Sonography Obstetrics and Gynecology, 3rd Ed. Baltimore: Lippincott Williams & Wilkins.

• Southern Alberta Institute of Technology materials, © 2016.