Why are umbilical Doppler findings abnormal in pregnancies affected by trisomy 18 (Edwards syndrome)?

Abnormal Umbilical Doppler Findings in Trisomy 18

Umbilical Doppler findings are frequently abnormal in trisomy 18 pregnancies due to placental insufficiency, which is a common feature of this chromosomal abnormality and contributes to the high prevalence of fetal growth restriction seen in these fetuses. 1

Pathophysiology of Abnormal Doppler in Trisomy 18

• Trisomy 18 is associated with a high prevalence of abnormal umbilical artery Doppler velocimetry, with studies showing abnormal pulsatility index (PI) in approximately 54% of affected fetuses 1
• The abnormality in umbilical artery blood flow reflects increased placental vascular resistance, which is likely due to abnormal placental development and vascularization associated with the chromosomal abnormality 1
• Placental insufficiency plays a significant role in the development of fetal growth restriction (FGR) in trisomy 18 fetuses, with up to 100% of affected fetuses being classified as FGR at birth 1, 2

Specific Doppler Abnormalities Observed

• Increased umbilical artery pulsatility index (PI) and resistance index (RI) beyond the 95th percentile for gestational age 1
• Abnormal half-peak systolic velocity deceleration time (hPSV-DT) in 58% of trisomy 18 fetuses 1
• Absent or reversed end-diastolic flow (AEDV or REDV) in the umbilical artery, which represents a severe form of abnormal Doppler and indicates significant placental dysfunction 3
• The prevalence of abnormal umbilical artery Doppler findings increases with advancing gestational age in trisomy 18 fetuses 1

Clinical Significance

• Abnormal umbilical Doppler findings in trisomy 18 correlate with poor perinatal outcomes, including high rates of intrauterine fetal demise (50%) and short postnatal survival 2
• Reversed end-diastolic flow in the umbilical artery has been documented as early as 10 weeks' gestation in trisomy 18 fetuses, suggesting very early placental dysfunction 3
• Fetal growth restriction is the most common sonographic finding in trisomy 18, present in nearly half of all cases at midpregnancy 4
• The combination of abnormal umbilical artery Doppler and fetal growth restriction should prompt consideration of chromosomal abnormalities, including trisomy 18 5

Placental Pathology

• Placentas from pregnancies with abnormal umbilical artery Doppler show distinct ultrastructural differences compared to normal placentas 6
• These abnormalities include increased number of blood vessels with tortuous courses and increased branching, reflecting abnormal angiogenesis 6
• The placental vascular abnormalities in trisomy 18 contribute to the placental insufficiency that leads to growth restriction and abnormal Doppler findings 1, 6

Clinical Management Implications

• When abnormal umbilical artery Doppler is detected, especially in early gestation or in combination with structural abnormalities, chromosomal testing should be offered 5
• The Society for Maternal-Fetal Medicine recommends that pregnant women be offered prenatal diagnostic testing with chromosomal microarray analysis (CMA) when unexplained isolated FGR is diagnosed at <32 weeks of gestation 5
• The presence of abnormal umbilical artery Doppler in combination with structural abnormalities significantly increases the likelihood of chromosomal abnormalities, including trisomy 18 5
• In cases where trisomy 18 is diagnosed, the poor prognosis should be considered in management decisions, as there were no perinatal survivors in some case series 1

Understanding the relationship between trisomy 18 and abnormal umbilical Doppler findings is crucial for early detection and appropriate counseling regarding this chromosomal abnormality with its associated poor prognosis.