A critical review of placental function evaluation near term using Doppler ratios

The evaluation of placental function near term remains a cornerstone of perinatal care, yet subtle dysfunction often escapes detection through conventional Doppler indices of the umbilical artery and middle cerebral artery. In this context, Doppler ratios-particularly the cerebroplacental ratio and the umbilicocerebral ratio-have emerged as promising tools for enhancing sensitivity in identifying early signs of fetal adaptation to hypoxia. This Clinical Opinion critically examines the rationale, clinical relevance, and limitations of cerebroplacental ratio and umbilicocerebral ratio as markers of placental insufficiency in the third trimester. Cerebroplacental ratio, which combines umbilical artery and middle cerebral artery impedance, has been shown to become abnormal before individual Doppler components, offering earlier recognition of compromised fetuses. Its use is supported in late-onset fetal growth restriction by several guidelines, including those of the International Society of Ultrasound in Obstetrics and Gynecology, although not by others such as the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine, which cite limited evidence of improved outcomes. The role of cerebroplacental ratio in appropriately grown fetuses remains controversial; while associations with adverse perinatal outcomes have been observed, the performance of this ratio in routine screening for adverse perinatal outcomes in the third trimester has been found to be poor, and randomized trials have not demonstrated clinical benefit from delivery based on cerebroplacental ratio alone. The current evidence is hindered by heterogeneity in cerebroplacental ratio thresholds, lack of standardization in measurement timing, and potential confounding by clinical interventions aimed at reducing adverse outcomes. Furthermore, most studies fail to isolate the predictive value of cerebroplacental ratio independently of abnormal umbilical artery or middle cerebral artery findings. While cerebroplacental ratio and umbilicocerebral ratio may contribute to risk stratification in late-onset fetal growth restriction, they should not currently be used in isolation to guide delivery timing. An ongoing trial is expected to clarify the role of umbilicocerebral ratio in guiding management decisions in late preterm fetal growth restriction. In the absence of growth restriction, routine assessment of these ratios in low-risk pregnancies may lead to unnecessary interventions without improving outcomes. However, when measured, abnormal ratios may still highlight increased risk and should be interpreted in conjunction with other ultrasound and clinical parameters.