Gestational-age interpretation of third-trimester fetal Doppler indices
Gestational-age–specific “normal” and “abnormal” values for fetal color Doppler indices in the third trimester are most commonly operationalized using gestational-age–specific centile charts (typically 5th, 10th, 50th, 90th, 95th) derived from uncomplicated pregnancies, with abnormality defined by being beyond the 95th centile for placental/umbilical resistance indices (umbilical artery PI, umbilical artery RI, MCA PI) and below the 5th centile for the cerebroplacental ratio (CPR). INTERGROWTH-21st umbilical artery PI chart, INTERGROWTH-21st umbilical artery RI chart, [1]
Umbilical artery pulsatility index (UA-PI)
Gestational-age–specific reference values
- Gestational-age–specific centile curves for UA-PI in the range of late second and third trimester are available from the INTERGROWTH-21st Project as an international reference chart with 5th, 10th, 50th, 90th, and 95th centile lines. INTERGROWTH-21st umbilical artery PI chart
Clinical interpretation
- UA-PI within the 5th to 95th centiles is interpreted as compatible with “normal” reference Doppler resistance for gestational age. INTERGROWTH-21st umbilical artery PI chart
- UA-PI above the 95th centile is interpreted as increased fetoplacental vascular resistance and is consistent with placental dysfunction phenotypes (commonly fetal growth restriction physiology when clinical context supports it). [1], INTERGROWTH-21st umbilical artery PI chart
Umbilical artery resistance index (UA-RI)
Gestational-age–specific reference values
- Gestational-age–specific centile curves for UA-RI in the third trimester are available from the INTERGROWTH-21st Project as an international reference chart with 5th, 10th, 50th, 90th, and 95th centile lines. INTERGROWTH-21st umbilical artery RI chart
Clinical interpretation
- UA-RI within the 5th to 95th centiles is interpreted as compatible with “normal” reference Doppler resistance for gestational age. INTERGROWTH-21st umbilical artery RI chart
- UA-RI above the 95th centile is interpreted as increased fetoplacental vascular resistance. INTERGROWTH-21st umbilical artery RI chart
Middle cerebral artery pulsatility index (MCA-PI)
Gestational-age–specific reference values
- Gestational-age–specific reference standards for the MCA-PI in low-risk pregnancies have been published, including reference frameworks that are used clinically alongside UA-PI to compute CPR (MCA-PI/UA-PI). [1]
Clinical interpretation
- MCA-PI within the gestational-age reference range supports an absence of marked redistribution physiology. [1]
- MCA-PI below the 5th centile is interpreted as reduced cerebral vascular resistance consistent with brain-sparing physiology when interpreted in combination with UA indices and fetal biometry/clinical context. [1]
Cerebroplacental ratio (CPR = MCA-PI / UA-PI)
Gestational-age–specific reference values
- Prescriptive third-trimester reference standards for CPR have been derived from low-risk pregnancies with validation, producing gestational-age–specific centiles including a 5th centile used for decision-making. [1]
Clinical interpretation
- CPR at or above the 5th centile is interpreted as not meeting the reference threshold for cerebroplacental redistribution. [1]
- CPR below the 5th centile is interpreted as abnormal and compatible with cerebral redistribution (brain-sparing) physiology in the appropriate clinical context. [1]
Ductus venosus (DV) waveform pattern (A-, S-, and V-wave patterns)
Reference pattern definition (qualitative)
DV Doppler is interpreted qualitatively by waveform components:
- S-wave: positive forward flow during ventricular systole. [2]
- V-wave (often described as v-descent component in DV waveform terminology): the post-systolic reduction in velocity. [3]
- A-wave: lowest point during atrial contraction that remains forward in normal physiology. [2], [3]
Clinical interpretation
- A normal DV pattern is interpreted when the atrial contraction A-wave is present and forward (not absent and not reversed). [2]
- An abnormal DV pattern is interpreted when the A-wave is absent or reversed, which reflects failing compensatory mechanisms and is associated with worse fetal outcomes in observational data. [3], [4]
- In abnormal physiology, the DV waveform typically shows other qualitative changes with progression, including diminished diastolic components and a “deeper” V-descent, alongside the abnormal A-wave. [4], [3]
Clinical integration and reporting rules
Which values define “abnormal”
- UA-PI abnormality is defined by being above the 95th centile on gestational-age–specific reference charts. INTERGROWTH-21st umbilical artery PI chart
- UA-RI abnormality is defined by being above the 95th centile on gestational-age–specific reference charts. INTERGROWTH-21st umbilical artery RI chart
- CPR abnormality is defined by being below the 5th centile on gestational-age–specific third-trimester reference standards. [1]
- DV abnormality is defined by absent or reversed A-wave during atrial contraction. [2], [3]
How interpretation should be applied in practice
- UA indices and CPR should be interpreted together because CPR operationalizes the relationship between cerebral and placental vascular resistance (MCA-PI relative to UA-PI). [1]
- DV should be interpreted as an indicator of cardiovascular compensation status rather than a stand-alone “growth restriction vs normal” test. [3]
Important methodological constraints that affect “reference normality”
- Gestational-age reference values assume standard insonation and waveform acquisition protocols because Doppler indices vary with sampling location, fetal state, and technical settings. [3]
- DV assessment should be performed with the fetus in a quiescent state to reduce misclassification of waveform components and the A-wave. [3]