Abnormal First Trimester Fetus
Anne Kennedy, MD
DIFFERENTIAL DIAGNOSIS
Common
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Increased Nuchal Translucency
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Cystic Hygroma
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Central Nervous System Anomalies, Severe
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Congenital Heart Defects
Less Common
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Absent Nasal Bone
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Gastroschisis
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Omphalocele
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Conjoined Twins
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Twin Reversed Arterial Perfusion
Rare but Important
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Autosomal Recessive Syndromes
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
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Must know normal developmental anatomy to avoid erroneous diagnosis of anomaly
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Brain: Infratentorial
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Rhombencephalon = precursor of cerebellum and brain stem
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Appears hypoechoic
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Not to be mistaken for posterior fossa cyst
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Brain: Supratentorial
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Presence of complete falx excludes alobar/semilobar holoprosencephaly
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Look for “butterfly” sign of choroids to exclude alobar/semilobar holoprosencephaly
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Skull vault
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Ossification visible by ∽ 12 weeks
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Abdomen
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Physiologic herniation of bowel is a normal embryological process
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Bowel leaves peritoneum → base of umbilical cord → rotates 270° then re-enters abdominal cavity
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Re-entry complete by 11.2 weeks gestational age
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Never normal to see liver in base of cord
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Limbs
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Limb buds develop by 9 weeks
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Femur can be measured by 13 weeks, visible earlier
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Hands and feet fully formed by 13 weeks
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Amniotic fluid
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Produced by membranes in first trimester
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Renal function does not account for majority of fluid volume until 16-17 weeks
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Cannot exclude renal agenesis, autosomal recessive polycystic kidney disease on basis of normal fluid in first trimester
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Helpful Clues for Common Diagnoses
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Increased Nuchal Translucency
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Check ductus venosus (DV) waveform
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Abnormal DV waveform = ↑ risk of adverse outcome even if chromosomes normal
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Trisomy 21
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Look for associated absent nasal bone, atrioventricular septal defect
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Cystic hygroma/skin edema in Down syndrome is truncal
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Trisomy 18
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Look for associated omphalocele, complex congenital heart disease
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Trisomy 13
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Look for associated alobar holoprosencephaly, cyclopia, proboscis
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Turner syndrome
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Fetuses often hydropic
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Look for “domed” extremity edema, Down syndrome edema more truncal
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Cystic Hygroma
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Look for internal septations on axial images
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Look for other stigmata of Down/Turner syndrome
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Central Nervous System Anomalies, Severe
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Exencephaly
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No skull vault echo
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Brain seen “too well” initially with eventual destruction
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Look for amniotic bands as etiology
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Anencephaly
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Look for “frog eye” appearance
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No skull or brain above orbits
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Occipital encephalocele
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Confirm defect from different scan planes to avoid confusion with cystic hygroma
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Alobar holoprosencephaly
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Butterfly sign of choroid will be absent
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Look for monoventricle/fused thalami
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Congenital Heart Defects
Helpful Clues for Less Common Diagnoses
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Absent Nasal Bone
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Midsagittal plane
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Normal appearance is 2 bright echoes, one from skin, shorter, brighter echo from bone
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Gastroschisis
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Bowel loops free in amniotic fluid, no surrounding membrane
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Cord inserted on abdominal wall, defect usually to the right
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Omphalocele
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Membrane bound defect
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Cord inserted at apex of defect
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Never normal to see liver involved in physiologic bowel herniation into base of cord
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Conjoined Twins
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Monochorionic monoamniotic gestation
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Fixed relationship of embryos/fetuses with contiguous skin covering
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Twin Reversed Arterial Perfusion
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TRAP: One normal “pump” twin
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One anomalous twin
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Diffuse truncal edema
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Often subcutaneous cysts in edematous tissues
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Absent or rudimentary cranial vault
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Always check direction of flow in umbilical artery of an anomalous twin
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Will be toward the anomalous fetus in TRAP
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Helpful Clues for Rare Diagnoses
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Autosomal Recessive Syndromes
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25% recurrence risk, early diagnosis allows intervention for poor outcome conditions
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Meckel Gruber Syndrome
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Occipital encephalocele, renal cystic dysplasia, polydactyly
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Achondrogenesis 1A, 1B
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Severe micromelia, poor spine ossification, hydrops
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Other Essential Information
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NT measurement technique
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Midsagittal scan plane
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Neutral head position
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Use (+) not (x) cursors
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Show amnion separate from nuchal skin
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Transvaginal ultrasound mandatory for adequate resolution of anomalies
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Normal first trimester scan does not exclude all anomalies
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Some entities change progressively over time
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Coarctation/aortic stenosis may not have significant hemodynamic effects until third trimester
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Aqueductal stenosis often presents as hydrocephalus in 3rd trimester
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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. |
![]() (Left) Pulsed Doppler ultrasound at 9 weeks shows abnormal ductus venosus flow with reversal during the A wave
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