Fetal Abnormalities Causing IUGR
Chromosomal disorders and congenital malformations are responsible for approximately 20% of all fetal growth restriction cases, making them the second most common cause after placental insufficiency. 1
Major Fetal Abnormality Categories
Chromosomal Abnormalities
- Chromosomal disorders account for approximately 6.4% of early-onset FGR cases, even without structural malformations. 2
- Autosomal trisomies are the most common chromosomal causes, including:
- Sex chromosome abnormalities such as Turner syndrome (45,X) and 47,XYY also cause IUGR 3
- Triploidy and other rare trisomies (trisomy 7, trisomy 16) have been documented 3
- Structural chromosomal abnormalities including translocations and deletions can result in growth restriction 3
Congenital Malformations
- Approximately 10% of fetuses with FGR have congenital anomalies 4
- Structural malformations contribute to the 20% of FGR cases attributed to fetal abnormalities 1
- A detailed fetal structural survey is essential as part of the diagnostic workup 4
Genetic Syndromes
- Genetic syndromes are more common etiologies earlier in gestation 4
- These conditions often present with symmetrical growth restriction 5
Intrauterine Infections
- Viral infections, particularly cytomegalovirus (CMV), are important fetal causes of IUGR 4, 6
- CMV is the primary infectious etiology to evaluate in early-onset FGR 2
- Infections are more commonly identified as causative factors in early-onset cases 4
Diagnostic Approach
Timing-Based Evaluation Strategy
For early-onset FGR (diagnosed <32 weeks):
- Offer prenatal diagnostic testing with chromosomal microarray analysis (CMA), which provides a 4-10% incremental diagnostic yield over standard karyotyping 2
- Perform detailed obstetrical ultrasound examination to identify structural abnormalities 2
- Consider PCR for CMV in women with unexplained FGR who elect diagnostic testing with amniocentesis 1, 2
- Do NOT screen for toxoplasmosis, rubella, or herpes in the absence of other risk factors 1
For late-onset FGR:
- Placental insufficiency predominates as the cause, especially related to hypertension and maternal vascular disease 4
- Chromosomal anomalies and infections are less likely but should still be considered if other features are present 4
Specific Testing Recommendations
- Chromosomal microarray analysis is recommended when unexplained isolated FGR is diagnosed at <32 weeks of gestation (GRADE 1C) 1
- CMA should also be offered when FGR is detected with fetal malformation, polyhydramnios, or both 2
- Doppler studies of maternal and fetal circulations are the most effective method to differentiate IUGR fetuses secondary to placental dysfunction from those secondary to aneuploidy, genetic syndromes, and intrauterine infections 5
Management Implications
Surveillance Adjustments
- Once fetal growth restriction is diagnosed, serial umbilical artery Doppler assessment should be performed to assess for deterioration 1
- The presence of chromosomal abnormalities or congenital malformations may alter the intensity and type of monitoring required 5
- Exclusion of structural and/or chromosomal anomalies helps guide appropriate counseling regarding prognosis 5
Common Pitfalls
- Failing to offer genetic testing for early-onset FGR (<32 weeks) misses the 6.4% of cases with chromosomal abnormalities even without visible structural defects 2
- Ordering unnecessary infectious disease screening (toxoplasmosis, rubella, herpes) without specific risk factors wastes resources and delays appropriate management 1
- Assuming all IUGR is placental in origin without performing a detailed anatomic survey misses the 10% with congenital anomalies 4
- Using standard karyotyping alone instead of CMA loses the 4-10% incremental diagnostic yield in detecting submicroscopic abnormalities 2