Abnormal First Trimester Fetus

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Increased Nuchal Translucency
Cystic Hygroma
Central Nervous System Anomalies, Severe
Congenital Heart Defects

Less Common

Absent Nasal Bone
Gastroschisis
Omphalocele
Conjoined Twins
Twin Reversed Arterial Perfusion

Rare but Important

Autosomal Recessive Syndromes

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Must know normal developmental anatomy to avoid erroneous diagnosis of anomaly
- Brain: Infratentorial
  - Rhombencephalon = precursor of cerebellum and brain stem
  - Appears hypoechoic
  - Not to be mistaken for posterior fossa cyst
- Brain: Supratentorial
  - Presence of complete falx excludes alobar/semilobar holoprosencephaly
  - Look for “butterfly” sign of choroids to exclude alobar/semilobar holoprosencephaly
- Skull vault
  - Ossification visible by ∽ 12 weeks
- Abdomen
  - Physiologic herniation of bowel is a normal embryological process
  - Bowel leaves peritoneum → base of umbilical cord → rotates 270° then re-enters abdominal cavity
  - Re-entry complete by 11.2 weeks gestational age
  - Never normal to see liver in base of cord
- Limbs
  - Limb buds develop by 9 weeks
  - Femur can be measured by 13 weeks, visible earlier
  - Hands and feet fully formed by 13 weeks
- Amniotic fluid
  - Produced by membranes in first trimester
  - Renal function does not account for majority of fluid volume until 16-17 weeks
  - Cannot exclude renal agenesis, autosomal recessive polycystic kidney disease on basis of normal fluid in first trimester

Helpful Clues for Common Diagnoses

Increased Nuchal Translucency
- Check ductus venosus (DV) waveform
- Abnormal DV waveform = ↑ risk of adverse outcome even if chromosomes normal
- Trisomy 21
  - Look for associated absent nasal bone, atrioventricular septal defect
  - Cystic hygroma/skin edema in Down syndrome is truncal
- Trisomy 18
  - Look for associated omphalocele, complex congenital heart disease
- Trisomy 13
  - Look for associated alobar holoprosencephaly, cyclopia, proboscis
- Turner syndrome
  - Fetuses often hydropic
  - Look for “domed” extremity edema, Down syndrome edema more truncal
Cystic Hygroma
- Look for internal septations on axial images
- Look for other stigmata of Down/Turner syndrome
Central Nervous System Anomalies, Severe
- Exencephaly
  - No skull vault echo
  - Brain seen “too well” initially with eventual destruction
  - Look for amniotic bands as etiology
- Anencephaly
  - Look for “frog eye” appearance
  - No skull or brain above orbits
- Occipital encephalocele
  - Confirm defect from different scan planes to avoid confusion with cystic hygroma
- Alobar holoprosencephaly
  - Butterfly sign of choroid will be absent
  - Look for monoventricle/fused thalami
Congenital Heart Defects
- Use color Doppler as well as high resolution grayscale imaging
- All fetuses with increased NT should have formal 2nd trimester echocardiography

Helpful Clues for Less Common Diagnoses

Absent Nasal Bone
- Midsagittal plane
- Normal appearance is 2 bright echoes, one from skin, shorter, brighter echo from bone
Gastroschisis
- Bowel loops free in amniotic fluid, no surrounding membrane
- Cord inserted on abdominal wall, defect usually to the right
Omphalocele
- Membrane bound defect
- Cord inserted at apex of defect
- Never normal to see liver involved in physiologic bowel herniation into base of cord
Conjoined Twins
- Monochorionic monoamniotic gestation
- Fixed relationship of embryos/fetuses with contiguous skin covering
Twin Reversed Arterial Perfusion
- TRAP: One normal “pump” twin
- One anomalous twin
  - Diffuse truncal edema
  - Often subcutaneous cysts in edematous tissues
  - Absent or rudimentary cranial vault
- Always check direction of flow in umbilical artery of an anomalous twin
  - Will be toward the anomalous fetus in TRAP

Helpful Clues for Rare Diagnoses

Autosomal Recessive Syndromes
- 25% recurrence risk, early diagnosis allows intervention for poor outcome conditions
- Meckel Gruber Syndrome
  - Occipital encephalocele, renal cystic dysplasia, polydactyly
- Achondrogenesis 1A, 1B
  - Severe micromelia, poor spine ossification, hydrops

Other Essential Information

NT measurement technique
- Midsagittal scan plane
- Neutral head position
- Use (+) not (x) cursors
- Show amnion separate from nuchal skin
Transvaginal ultrasound mandatory for adequate resolution of anomalies
Normal first trimester scan does not exclude all anomalies
- Some entities change progressively over time
  - Coarctation/aortic stenosis may not have significant hemodynamic effects until third trimester
  - Aqueductal stenosis often presents as hydrocephalus in 3rd trimester

Image Gallery

Sagittal ultrasound shows typical increased nuchal translucency (calipers) of 3 mm in a fetus with trisomy 21. In the second trimester nuchal fold skin thickening and brachycephaly developed.

Sagittal ultrasound at 13 weeks shows increased nuchal translucency

and pleural effusions (not shown). Subsequently a cystic hygroma developed and genetic amniocentesis revealed Turner syndrome.

(Left) Pulsed Doppler ultrasound at 9 weeks shows abnormal ductus venosus flow with reversal during the A wave

. Follow-up at 13 weeks showed ↑ nuchal translucency. At birth infant had a non-lethal, short-limbed, skeletal dysplasia. (Right) Axial transvaginal ultrasound in a fetus with cystic hygroma