Development of the Fetus, Placenta, Birth Defects & Prenatal Diagnosis – Comprehensive Notes

Page 1 – Scope of Lecture VII

• Topics: Development of the fetus, fetal membranes & placenta, birth defects, prenatal diagnosis (General Embryology VII).
• Integrates morphologic events, functional maturation, pathology, and diagnostic strategies.

Page 2 – Timeline of Human Development

• Two major developmental phases:
– Embryonic period: fertilization → end of $8^{th}$ week (≈ $56$ days).
– Fetal period: $8^{th}$ week → birth.
• Conventional trimester system:
– $1^{st}$ trimester: weeks $0!–!12$.
– $2^{nd}$ trimester: weeks $13!–!27$.
– $3^{rd}$ trimester: weeks $28!–!38$(40).
• Key fetal-age checkpoints (crown–rump length CRL in cm): $12,16,20,24,28,32,36,38$.
• Viability steadily rises; average term birth ≈ $38$ weeks post-fertilization (≈ $40$ gestational wks).

Page 3 – Hallmarks of the Fetal Period

• Tissue/organ maturation replaces organogenesis.
• Rapid somatic growth (length then weight).
• Disproportion correction of head-to-body ratio:
– At $12^{th}$ week $\rightarrow$ head : body ≈ $1:1$.
– At mid-gestation $\rightarrow$ ≈ $1:3$.
– At birth $\rightarrow$ ≈ $1:4$.
• Functional practice (muscle activity, swallowing, breathing movements) begins.

Page 4 – 11-Week Fetus (Early $1^{st}$ Trimester)

• Size: $5!–!7\ \text{cm}$ CRL.
• Umbilical cord shows physiologic swelling (contains returning mid-gut loop).
• Digits fully separated; nails beginning.
• Skull contour irregular (future fontanelles).
• Eyes still positioned ventrally and widely separated (optic axis migration incomplete).

Page 5 – 12-Week Fetus (End of $1^{st}$ Trimester)

• Size: ≈ $8\ \text{cm}$ CRL.
• Primary ossification centers visible on radiograph in long bones and cranial vault.
• External genitalia morphologically distinct $\rightarrow$ sex determination by high-resolution US.
• Dermal vascular network apparent.
• Physiologic herniation of gut retracts into abdominal cavity.
• Spontaneous muscular activity (limb kicks) measurable.

Page 6 – 18-Week Fetus (Mid $2^{nd}$ Trimester)

• Size: ≈ $15\ \text{cm}$ CRL.
• Eyebrows & scalp hair (lanugo & emerging terminal hair).
• Auricles reach definitive cranio-caudal level (align with eyes/nose).
• Quickening (first maternal perception of movement) typically occurs.

Page 7 – 28-Week Fetus (Early $3^{rd}$ Trimester)

• Size: ≈ $27\ \text{cm}$ CRL.
• Well-rounded physique (accumulating subcutaneous fat).
• Most organ systems functional except:
– Respiratory (type II pneumatocyte surfactant borderline).
– Central nervous system (thermoregulation, breathing rhythm immature).
• Delivery at $7$ months (≈ $28$ weeks) ⇒ $90\%$ survival with modern neonatal care.

Page 8 – Fetal Membranes & Placenta: General Concepts

• Placenta = fetomaternal organ comprising:
– Fetal component: chorion (trophoblast + extra-embryonic mesoderm).
– Maternal component: decidua (modified endometrium).
• Core functions:

1. Protection (immune barrier; IgG transfer).

2. Nutrition (glucose, amino acids, lipids, micronutrients).

3. Respiration (O$<em>2$ uptake, CO$</em>2$ release).

4. Excretion (urea, bilirubin).

5. Endocrine: $hCG$, human placental lactogen, estrogens, progesterone.
   • Nutrient demand grows with fetal mass ⇒ surface area increases via chorionic villus elaboration.

Page 9 – Supplemental Learning Links

• Implantation & placental formation (Study.com video).
• Placenta/fetus structure-function (Study.com video).
• Encourage visualizing 3-D relationships.

Page 10 – $7.5$-Day Blastocyst (Implantation Begins)

• Trophoblast differentiates:
– Syncytiotrophoblast: invasive multinucleate layer eroding endometrium.
– Cytotrophoblast: inner mononucleate cells (mitotic).
• Amnioblasts line emerging amniotic cavity above epiblast.
• Hypoblast lines blastocyst cavity (primitive endoderm).
• Uterine glands & maternal vessels start to interface with trophoblast.

Page 11 – $9$-Day Blastocyst

• Trophoblastic lacunae form within syncytiotrophoblast (future intervillous spaces).
• Exocoelomic (Heuser’s) membrane + hypoblast generate primitive yolk sac.
• Early maternal blood engorgement of lacunae (precursor to uteroplacental circulation).
• Fibrin coagulum seals implantation defect at surface epithelium.

Page 12 – $12$-Day Blastocyst

• Maternal sinusoids anastomose with lacunae ⇒ primordial uteroplacental flow.
• Extra-embryonic mesoderm splits into:
– Somatic (lining trophoblast + amnion).
– Splanchnic (covering yolk sac).
• New fluid cavity: extra-embryonic (chorionic) cavity surrounds embryo except at connecting stalk.

Page 13 – $13$-Day Blastocyst

• Secondary yolk sac forms (smaller, definitive).
• Chorionic cavity enlarges; trophoblastic villi initiate.
• Oropharyngeal membrane = cranial bilaminar disc landmark (future mouth).
• Exocoelomic cysts may persist within chorionic cavity.

Page 14 – Villi Maturation

• Primary villus (≈ day $13$): cytotrophoblast core covered by syncytiotrophoblast.
• Secondary villus (≈ day $16$): extra-embryonic mesoderm invades core.
• Tertiary villus (≈ day $21$): mesoderm core vascularizes → villous capillaries connect to embryonic heart.
• Intervillous spaces (maternal blood) surround villi, maximizing exchange surface.

Page 15 – Maternal–Fetal Circulation (Beginning $2^{nd}$ Month)

• Spiral arteries deliver oxygenated maternal blood into intervillous space.
• Outer cytotrophoblast shell anchors chorionic plate to decidua basalis.
• Venous return via endometrial veins.
• Chorionic plate = fetal side; continuous with chorionic cavity.
• Decidua capsularis overlies expanding chorion toward uterine lumen.

Page 16 – Placental Micro-Cross-Section

• Layers (maternal → fetal):

1. Endometrium / decidua basalis.

2. Intervillous space (maternal blood).

3. Syncytiotrophoblast (barrier + endocrine).

4. Cytotrophoblast (diminishes by term).

5. Mesodermal core with capillaries (fetal blood).
   • Connecting stalk (future umbilical cord) links embryo to chorionic plate.

Page 17 – Spatial Relations of Membranes

• End $2^{nd}$ month: chorionic cavity large; amnion small.
• End $3^{rd}$ month: amnion expands & fuses with chorion ⇒ obliterates chorionic cavity.
• Decidua parietalis (uterine wall) fuses with chorion laeve (smooth chorion).
• Chorion frondosum (villous region facing decidua basalis) persists as definitive placenta.
• Remnant yolk sac diminishes.

Page 18 – Birth Defects: Definition & Epidemiology

• Synonyms: congenital malformation/anomaly.
• Include structural, functional, metabolic, behavioral disorders present at birth.
• Leading cause of infant mortality.
• Etiologic fractions:
– Genetic (chromosomal/gene) ≈ $28\%$.
– Environmental (teratogens) ≈ $3!–!4\%$.
– Multifactorial (gene + environment) ≈ $20!–!25\%$.
– Unknown ≈ $40!–!45\%$.

Page 19 – Teratogenic Sensitivity Curve

• Greatest risk window: embryonic period (organogenesis, weeks $3!–!8$).
• Fetal period still vulnerable but mainly to functional defects.
• First prenatal visit ideally precedes conception; yet often at $8$ wks or later (gap highlights preventive need).
• Risk declines toward term but never reaches $0$ until parturition.

Page 20 – Environmental Teratogens & Historic Proof

• Categories:

1. Infectious: rubella, cytomegalovirus, Zika.

2. Radiation: ionizing doses > $0.1$ Gy.

3. Chemicals/drugs: alcohol, isotretinoin, thalidomide (1961 limb reduction revelation).

4. Nutritional: folate deficiency ⇒ neural tube defects.

5. Maternal disease: diabetes mellitus, PKU.
   • Rubella 1941 (Gregg) linked maternal infection in the $1^{st}$ trimester to cataracts & heart defects.

Page 21 – National Prevalence Study (CDC, NBDPN 2010)

• Covers U.S. live births $2004!–!2006$; methodology adjusts for surveillance heterogeneity.
• Serves as baseline for public-health planning, research prioritization.

Page 22 – Selected Birth-Defect Prevalence (Adjusted per $10{,}000$ Live Births)

• CNS:
– Anencephaly $2.06$ ($\approx1/4{,}859$).
– Spina bifida $3.50$.
• Cardiac:
– Transposition of great arteries $3.00$.
– Tetralogy of Fallot $3.97$.
– AV septal defect $4.71$.
– Hypoplastic left heart $2.30$.
• Orofacial:
– Cleft palate only $6.35$.
– Cleft lip ± palate $10.63$.
• GI:
– Esophageal atresia $2.17$.
– Rectal/large-intestinal atresia $4.68$.
• Musculoskeletal:
– Upper-limb reduction $3.49$.
– Gastroschisis $4.49$ (rising trend, maternal age correlation).
• Chromosomal (age-adjusted):
– Trisomy 21 $14.47$ ($1/691$).
– Trisomy 18 $2.66$.
– Trisomy 13 $1.26$.

Page 23 – Visual Examples of Defects

• Illustrated conditions: Turner syndrome (45,X), cleft lip, phocomelia (thalidomide), right ventricular hypertrophy, Treacher Collins syndrome, spina bifida, tetralogy of Fallot, aortic outflow anomalies, ventricular septal defect.
• Emphasize recognition of phenotypic patterns and pathophysiology links.

Page 24 – Prenatal Diagnosis: Ultrasonography

• Non-invasive, uses high-frequency sound.
• Approaches: transabdominal (standard) vs transvaginal (earlier, higher resolution).
• Routine anatomy scan at $18!–!20$ wks evaluates:
– Growth parameters (BPD, FL, AC).
– Placental position, amniotic fluid volume.
– Structural anomalies (NTDs, heart, kidneys, limbs).
• Serial scans aid in monitoring IUGR, twins, previa.

Page 25 – Maternal Serum Screening (MSS)

• Focus marker α-fetoprotein (AFP):
– Fetal liver glycoprotein; peaks $≈14$ wks in fetal serum.
– Leaks into maternal blood via placenta; rises until $≈30$ wks, then declines.
• Test window: $15!–!20$ wks (quad screen).
• High AFP ⇒ open NTDs, ventral wall defects, underestimated gestational age.
• Low AFP (with high hCG, low uE3, ↑ inhibin A) ⇒ increased risk of trisomy 21.
• MSS is screening, not diagnostic; dictates further testing.

Page 26 – Amniocentesis

• Procedure: transabdominal aspiration of $20!–!30$ ml amniotic fluid (weeks $15!–!20$).
• Analyses:
– Biochemical: AFP, acetylcholinesterase.
– Cytogenetic: karyotype (culture $≈2$ wks).
– Molecular: PCR for single-gene disorders (e.g., CF, SMA).
• Risks: miscarriage (~$0.1!–!0.3\%$), Rh sensitization, infection; ultrasound guidance minimizes complications.

Page 27 – Chorionic Villus Sampling (CVS)

• Sampling: $5!–!30$ mg chorionic tissue via transcervical or transabdominal route earlier (weeks $10!–!12$).
• Advantages: results in few days; allows early decision-making.
• Applications: chromosomal analysis, DNA mutations, metabolic assays.
• Limitations: cannot assess neural-tube defects (no AFP); slightly higher miscarriage risk (≈ $0.5\%$).
• Local limb reduction risk once suggested but now considered negligible with modern technique.

Study these page-wise notes alongside diagrams for holistic mastery. Numerical data and timelines have been preserved in LaTeX for precision.