Fetal Skeleton

Question 1

What are the main components of the fetal skeletal anatomy?

Answer 1

Axial Skeleton:
* Skull: Cranium, frontal, temporal, occipital bones
* Facial Bones: Orbits, maxilla, mandible, bony nasal septum
* Spine

Appendicular Skeleton:
* Upper and lower extremity bones
* Pelvis

Blue: Axial Red: Appendicular

The axial skeleton supports the head and trunk.

Question 2

When can various upper extremity bones be visualized on fetal ultrasound?

Answer 2

• Scapulae and clavicles: Seen by 7 menstrual weeks
• Radius and ulna: Visible by mid-second trimester
• Hand bones: Visible by mid-second trimester

Early visualization is crucial for assessing fetal development.

Question 3

Answer 3

Question 4

What is rhizomelia?

Answer 4

Shortening of the proximal segment of an extremity

Commonly affects the humerus or femur.

Question 5

What is mesomelia?

Answer 5

Shortening of the distal segments of an extremity

Affects radius/ulna or tib/fib.

Question 6

What is micromelia?

Answer 6

All segments of the extremities are present but short

It affects both proximal and distal segments.

Question 7

What is amelia?

Answer 7

Absence of an extremity

Causes: Genetic mutations, environmental factors (thaildomyde), vascular accidents amniotic band

Question 8

What is meromelia?

Tetra-amelia syndrome characterized by the absence of all four limbs

Answer 8

Partial absence of a limb or limbs

Tetra-amelia syndrome is characterized by the absence of all four limbs

Causes: Genetic mutations, environmental factors (thaildomyde), vascular accidents amniotic band

Question 9

What is polydactyly?

Answer 9

Presence of extra digits on fetal hands or feet

Associated with:
* Short polydactyly syndrome
* Asphyxiating thoracic dysplasia
* Aneuploidies such as trisomy 13, 18, 21

One of the most common hand anomalies and may occur as an isolated finding or as part of a syndrome

Question 10

What is syndactyly?

Answer 10

Soft tissue or bony fusion of digits

Normal (top) vs Abnormal (bottom)

It can affect fingers or toes.

Question 11

What is phocomelia?

Answer 11

A rare congenital deformity where hands or feet are attached close to the trunk and limbs underdeveloped or absent

Often associated with** thalidomide exposure.**

Question 12

What should be measured in addition to the femur?

Answer 12

Humerus, radius, ulna, tibia, fibula

Helps in assessing gestational age and skeletal abnormalities.

Question 13

When can fetal limbs first be seen on ultrasound, and when are they fully developed?

Answer 13

• Fetal limb buds can be seen as early as 8 weeks with endovaginal ultrasound
• Lower limb buds appear before upper limb buds
• Limbs are fully developed and primary ossification centers are visible by 10 weeks

Lower limb buds typically visualize before upper ones.

Question 14

Why is the fetal femur commonly measured in obstetric ultrasound, and how does its growth change throughout pregnancy?

Answer 14

• The femur is the most commonly measured long bone to assess fetal growth.

Growth rate:
* \~3 mm/week from 14 to 28 weeks
* \~1 mm/week during the third trimester

Measurement accuracy decreases with gestational age:
* ±1 week in the second trimester
* ±3.5 weeks at term

Femur length varies based on maternal height and weight.

Question 15

What are the key features of fetal hands and feet in evaluation?

Answer 15

• Presence or Absence: Always document the presence of hands and feet.

Hand Positioning:
* Slight flexion is normal.
* Hands should move between flexion and extension.
* Fixed hand position is abnormal.

Feet and Legs Relationship:
* Ensure proper alignment to rule out clubfoot.
* You should not see the lower leg with the bottom of the feet in the same plane.
* Normal view of feet appears in a “hang ten” image — feet visible without lower legs.

Question 16

What is dysostosis?

Answer 16

A group of skeletal disorders where individual bones or groups of bones develop abnormally.

Can occur singly or in combinations with congenital anomalies.

Question 17

What is the most common skeletal anomaly detected during routine OB sono exams?

Answer 17

Club foot

Can be an isolated finding or part of broader anatomical abnormalities.

Question 18

What causes clubfoot (talipes equinovarus), and how is it typically treated?

Answer 18

• Caused by shortened Achilles tendon
• Foot turns inward and downward
• More common in boys (2x more likely)

Requires treatment:
* Phase 1: Casting
* Phase 2: Bracing

Question 19

What is a rocker bottom foot, and which conditions is it associated with?

Answer 19

It’s an abnormal curvature of the sole, seen on a sagittal ultrasound view.

It is associated with:
* Trisomy 13
* Trisomy 18
* Spina bifida

Question 20

What foot abnormalities can be detected in a plantar view, and what are they associated with?

Answer 20

• Polydactyly – extra digits
• Syndactyly – fused digits
• Sandal gap – widened space between the first and second toes

Associated with:
* Trisomy 21 (Down syndrome)
* Triploidy

Question 21

Know the anatomy and sonographic appearnace of the fetal hand.

Answer 21

Question 22

What is a radial ray anomaly?

Answer 22

Partial to complete absence of the radius, often associated with wrist and thumb abnormalities

Associated Abnormalities:
* **Trisomy 18 (Edwards Syndrome) **
* Amniotic band syndrome
* Holt-Oram Syndrome
* VACTERL association (vertebral
* Anal atresia
* Cardiac defects
* Tracheoesphageal fistula
* Renal anomalies
* Limb abnormalities)

Question 23

What hand abnormality is associated with Trisomy 21 and affects the 5th digit?

Answer 23

• Hypoplasia of the proximal phalanx of the 5th digit
• Clinodactyly – curvature or abnormal angulation of the 5th finger
• Commonly seen in Trisomy 21 (Down syndrome)

Question 24

What are osteochondrodysplasias?

Answer 24

Skeletal anomalies from abnormal growth or development of cartilage and bone

Also known as skeletal dysplasias or short limb dysplasias.

Question 25

What is thanatophoric dwarfism?

Answer 25

* **Most common lethal skeletal dysplasia** * Caused by **respiratory failure** due to **pulmonary hypoplasia** * **"Thanatophoric"** means **"death bearing"** in Greek * Occurs **twice as often in males** * **Etiology**: Autosomal dominant mutations in **FGFR3** (fibroblast growth factor receptor 3) * **Two types**: Type I and Type II * **Prognosis**: Poor — stillbirth or death shortly after birth ## Footnote Characterized by curved or bowed long bones and poor prognosis.

Question 26

What are the sonographic findings of **Thanatophoric Dysplasia**

Answer 26

* **Most common lethal skeletal dysplasia** * Caused by **respiratory failure** due to **pulmonary hypoplasia** * **"Thanatophoric"** means **"death bearing"** in Greek * Occurs **twice as often in males** * **Etiology**: Autosomal dominant mutations in **FGFR3** (fibroblast growth factor receptor 3) * **Two types**: Type I and Type II * **Prognosis**: Poor — stillbirth or death shortly after birth ## Footnote **Associated findings:** - Macrocephaly - Hydrocephalus - Patent ductus arteriosus - Atrial septal defect - Horseshoe kidney - Hydronephrosis - Imperforate anu

Question 27

What is achondroplasia and its sonographic findings?

Answer 27

* **Most common non-lethal type ofdwarfism** * Affects **males and females equally** * Caused by **autosomal dominant** mutation in the **FGFR3 gene** * A **genetic disorder** impacting normal skeletal growth *Sonographic findings:* * **Rhizomelic limb bowing** (shortening of proximal limbs) * **Frontal bossing** with **low nasal bridge** * **“Trident” hand** (increased space between 3rd & 4th digits) * **Macrocephaly** * May have **hydrocephaly** * **Good prognosis** – normal intelligence and life expectancy

Question 28

What is achondrogenesis?

Answer 28

Rare and lethal condition characterized by **hypomineralization and absent ossification of vertebral bodies** ## Footnote Associated with severe limb shortening and other anomalies.

Question 29

What are the sonographic features of Achondrogenensis?

Answer 29

* Narrow thorax * Bone fractures * **Lack of vertebral ossification** * Small chest * Large head with slightly decreased cranial ossification * Severely shortened limbs (usually all limbs involved)

Question 30

What characterizes short rib-polydactyly syndrome?

Answer 30

Short limbs, excessive number of digits, extremely narrow thorax **Sonographic findings:** * Short and horizontal ribs with a narrow thorax * Hypomineralization * Polydactyly * Syndactyly * Gastrointestinal/genital/urogenital malformations * Cleft lip/palate * Polyhydramnios * Micromelia ## Footnote It is a rare lethal autosomal recessive disorder.

Question 31

What is campomelic dysplasia?

Answer 31

A severe disorder affecting skeletal and reproductive system development **Sonographic Findings:** * Severe bowing of long bones especially in the** lower extremity** * Narrowed thorax * Hypoplastic scapulae * Associated hydronephrosis or hydrocephalus ## Footnote Often life-threatening in the newborn period. * Rare osteochondrodysplasia * “Bent limb” * Sharp mid-femoral ben

Question 32

What is Asphyxiating Thoracic Dystrophy and what are its key sonographic features?

Answer 32

* Also known as Jeune thoracic dystrophy or Jeune Syndrome * A lethal short-limb skeletal dysplasia * Sometimes classified as a type of short rib-polydactyly syndrome **Sonographic signs:** * Shortened long bones * Narrowed thorax * Cystic dysplastic kidneys * Polydactyly ## Footnote It is classified as a type of short rib-polydactyly syndrome.

Question 33

What are the key features of Chondroectodermal Dysplasia (Ellis-van Creveld Syndrome)?

Answer 33

* Also known as Ellis-van Creveld Syndrome * Polydactyly * Short limbs * Narrow thorax * Heart malformations * Dysplastic nails and teeth * Abnormal upper lip * More common in isolated communities with inbreeding, e.g., Old-Order Amish

Question 34

What are the sonographic findings and prognosis of Rhizomelic Chondrodysplasia Punctata?

Answer 34

**Sono signs:** * Symmetric shortening of proximal long bones (humerus/femur) * Low nasal bridge * Proximal limb shortening (rhizomelia) * Poor prognosis: most affected infants die before age 2

Question 35

What are the characteristics and inheritance patterns of Osteogenesis Imperfecta (OI) types I and II?

Answer 35

Also known as brittle bone disease, characterized by bone fragility due to collagen formation defects **OI Type I:** * Mildest and most common form (≈50%) * Few fractures and deformities * Inherited as autosomal dominant **OI Type II:** * Most severe form * Very short limbs, small chest, soft skull * Often born with fractures * Poorly developed lungs, low birth weight * Usually fatal within weeks of birth * Inherited as autosomal dominant ## Footnote Types I, II, and IV are inherited as autosomal dominant traits.

Question 36

What are the key sonographic and clinical features of Osteogenesis Imperfecta?

Answer 36

* **Main feature:** Hypomineralization of bone → skeletal fragility * **In utero signs:** Fractures and long-bone shortening * **Neonatal risks:** Intracranial hemorrhage, stillbirth due to delivery trauma * **Prognosis:** No known treatment available

Question 37

What abnormalities are commonly associated with Osteogenesis Imperfecta?

Answer 37

* Intrauterine Growth Restriction (IUGR) * Macrocephaly * Umbilical hernia * Clear visualization of the upside of the brain may suggest OI

Question 38

What are the key features of Hypophosphatasia and its sonographic findings?

Answer 38

* Metabolic disorder causing bone demineralization * Deficiency of serum alkaline phosphatase * Similar sonographic findings to osteogenesis imperfecta * Skeletal abnormalities: weakened, softened bones * Associated with polyhydramnios and IUGR