Page 2 of 3

AN80.1-7 | Fetal membranes — Self-Directed Learning

Part 1: Fetal Membranes — Chorion, Amnion, Yolk Sac, Allantois, Decidua (AN80.1)

1. Chorion

Formation: The chorion is formed by the trophoblast + extraembryonic somatic mesoderm. Initially all of the chorion has villi (chorionic villi = primary, secondary, tertiary stages).

Types of chorion:
• Chorion frondosum (bushy chorion) — the part of the chorion facing the decidua basalis (uterine wall under the embryo); villi proliferate extensively → becomes the fetal part of the placenta
• Chorion laeve (smooth chorion) — the rest of the chorion; villi atrophy → smooth, forms the outer chorionic membrane (no villi)

Functions:
• Produces hCG (human chorionic gonadotropin) — maintains corpus luteum → progesterone production → maintains pregnancy; basis of pregnancy tests (hCG detectable from Day 8–10 post-fertilisation)
• Produces HPL (human placental lactogen) — insulin resistance, lipolysis → provides nutrients to fetus

2. Amnion

Formation: The amnion arises from the epiblast (amniogenic cells) surrounding the amniotic cavity from Day 8.

Amniotic fluid:
• Composition: 98% water + fetal cells, proteins, vernix, lanugo
• Volume: 50 mL at 12 weeks → 800–1000 mL at 36 weeks → decreases to ~500–600 mL at 40 weeks
• Sources: Fetal urine (primary source after 16 weeks), fluid from fetal lung, amnion secretion
• Removal: Fetal swallowing (500–600 mL/day), absorption across amnion, intramembranous absorption

Functions of amniotic fluid:

Function	Details
Cushion against trauma	Protects fetus from external mechanical injury
Temperature regulation	Thermostat for fetus
Allows fetal movement	Essential for limb and muscle development
Prevents adhesions	Prevents body parts fusing to amnion
Lung development	Fetal breathing movements required for alveolar development
Infection barrier	Antimicrobial properties

Polyhydramnios (>2 L):
• Causes: Anencephaly (no swallowing), oesophageal/duodenal atresia (cannot swallow), maternal diabetes, fetal hydrops

Oligohydramnios (<300 mL at term):
• Causes: Renal agenesis/dysplasia (no urine production → Potter sequence: renal agenesis + oligohydramnios → pulmonary hypoplasia + limb deformities + flat facies), posterior urethral valves (bladder outlet obstruction), post-dates, IUGR

3. Yolk Sac (Secondary)
• Forms from hypoblast proliferation
• Functions: Early haematopoiesis (blood formation before the liver); primordial germ cell origin (yolk sac → gonads); source of early embryonic nutrition
• Fate: Communicates with gut via the vitello-intestinal (omphalomesenteric) duct; normally obliterates by the 6th week
• Clinical: Persistence of the duct → Meckel's diverticulum (most common congenital gut anomaly): remnant of omphalomesenteric duct on the ileum, 2 feet from ileocaecal valve, 2 cm long, 2% of population, presents by age 2

4. Allantois
• A diverticulum from the caudal yolk sac/hindgut that extends into the umbilical cord
• Functions: Contributes to blood vessel formation in early placenta; extraembryonic haematopoiesis
• Fate: Vestigial in humans; the intraembryonic portion → urachus (connecting bladder to umbilicus)
• Clinical: Patent urachus → urine drains from umbilicus; urachal cyst; urachal carcinoma (rare)

5. Decidua
• The endometrium that has undergone decidual reaction (enlarged stromal cells = decidual cells)

Region	Location	Fate
Decidua basalis	Under the embryo, between implantation site and myometrium	Forms the maternal part of the placenta
Decidua capsularis	Over the embryo (covering it toward the uterine lumen)	Atrophies as uterus expands
Decidua parietalis	Remaining uterine wall	Shed at delivery with placenta

Placenta Praevia: Implantation occurs in the lower uterine segment (over the decidua basalis of the lower segment) → placenta covers the internal os → painless antepartum haemorrhage as lower segment forms and dilates in late pregnancy.

Part 2: Umbilical Cord — Formation and Structure (AN80.2)

Formation of the Umbilical Cord

The umbilical cord forms from the connecting stalk (extraembryonic mesoderm) that attaches the embryo to the chorion. As the embryo folds (lateral and head/tail folding) in the 4th week:
• The connecting stalk + yolk sac stalk + allantois are incorporated into the umbilical cord
• The cord elongates progressively as the embryo grows within the expanding amniotic cavity

Structure of the Normal Umbilical Cord:

Component	Details
Length	50–60 cm (normal); <35 cm = short cord; >80 cm = long cord
Diameter	~1–2 cm
Vessels	2 umbilical arteries + 1 umbilical vein (rule of 2:1 — arteries:vein)
Wharton's jelly	Gelatinous mucoid connective tissue (proteoglycans + water); no nerves
Surface	Covered by amnion (thin, translucent)
Spirals	Left-handed helical coiling (protective against compression)

Blood flow direction:
• Umbilical arteries (×2): Carry deoxygenated blood from the fetus to the placenta (arteries carry blood AWAY from the heart/fetus, regardless of oxygen content)
• Umbilical vein (×1): Carries oxygenated blood from the placenta to the fetal liver (via left branch of portal vein and ductus venosus to IVC)

Clinical Correlations:
• Single umbilical artery (SUA): Only 1 artery + 1 vein; associated with congenital anomalies in 30% (cardiac, renal, chromosomal — especially trisomy 18); requires detailed fetal echocardiogram and anomaly scan
• Short cord: Causes abruptio placentae, cord rupture, or failure to descend in labour
• Long cord: Predisposes to cord prolapse and cord entanglement (nuchal cord — around baby's neck)
• Cord prolapse: Umbilical cord falls below the presenting part → compressed between fetus and pelvis → fetal bradycardia → obstetric emergency; management = emergency CS
• Nuchal cord: Umbilical cord around the neck; present in ~25% of deliveries; usually no consequence; tight nuchal cord → fetal distress
• Velamentous insertion: Cord inserts into the membranes rather than the placenta → vessels run unprotected → vasa praevia (vessels over internal os → rupture with membrane rupture → acute fetal haemorrhage and death)

Part 3: Placenta — Formation, Structure, Functions, Feto-Maternal Circulation (AN80.3)

Formation of the Placenta

The placenta develops from two sources:
• Fetal part: Chorion frondosum (chorionic villi + extraembryonic mesoderm)
• Maternal part: Decidua basalis

Stages of villous development:
1. Primary villi (Day 13): Trophoblast columns without mesoderm
2. Secondary villi (3rd week): Extraembryonic mesoderm core enters villi
3. Tertiary villi (3rd week onward): Blood vessels form within the mesodermal core → functional vascularised villi

Cotyledons:
The mature placenta is divided into 15–30 cotyledons (maternal septa from decidua basalis divide the placenta into lobes). Each cotyledon contains 1–2 stem (anchoring) villi and many free floating villi.

Gross features at term:
• Diameter: 15–20 cm
• Thickness: 2.5–3 cm
• Weight: ~500 g (~1/6th of baby's weight)
• Fetal surface: Smooth, shiny, covered by amnion; cord inserts centrally (normally)
• Maternal surface: Rough, lobulated (15–30 cotyledons), greyish-red

Physiological Functions of the Placenta:

Function	Mechanism
Respiration	O₂ diffuses from maternal blood → fetal blood; CO₂ in reverse; Fetal Hb (HbF) has HIGHER O₂ affinity than adult HbA → facilitates O₂ transfer
Nutrition	Glucose (facilitated diffusion), amino acids (active transport), fats (diffusion), water and ions
Excretion	Urea, creatinine, bilirubin transferred to maternal circulation for excretion
Hormone production	hCG (1st trimester), hPL, oestrogen (oestriol, from DHEAS in fetal adrenal), progesterone
Immune protection	Transfers IgG (passive immunity) → protects neonate for 3–6 months
Barrier	Prevents large molecules, bacteria from crossing

Feto-Maternal Circulation:

The human placenta is haemochorial — maternal blood directly bathes the chorionic villi (no maternal vessel wall between blood and trophoblast).

Intervillous space:
• Syncytiotrophoblast invades and erodes maternal spiral arterioles → maternal blood jets into the intervillous space
• Oxygen, glucose, IgG etc. diffuse across the placental barrier into fetal blood vessels within the villi
• Fetal blood enters villi via umbilical arteries (deoxygenated) → capillaries in tertiary villi → oxygenated blood exits via umbilical vein

Placental Barrier (Layers from maternal blood to fetal blood):
1. Syncytiotrophoblast (outermost; covers all villi)
2. Cytotrophoblast (Langhans cells; disappear by 4th month → barrier thins)
3. Connective tissue of villus (Wharton's jelly)
4. Endothelium of fetal capillary

The barrier THINS as pregnancy progresses (from ~25 µm in early pregnancy to 2–4 µm at term) → improving transfer efficiency.

What CROSSES the placental barrier:
• O₂, CO₂, water, glucose, amino acids, fatty acids, electrolytes
• IgG (passive immunity)
• Drugs: alcohol, most anaesthetics, steroids, heparin does NOT cross
• Infections: Rubella, CMV, Toxoplasma, HIV (vertical transmission) — all cross
• Teratogens: thalidomide, valproate, warfarin, isotretinoin

What does NOT cross:
• Bacteria (generally)
• Heparin (large molecule)
• Maternal RBCs (normally) — Rh-negative mothers can be sensitised if feto-maternal haemorrhage occurs