In an earlier publication by the Prospective Observational Trial to Optimize Pediatric Health in Intrauterine Growth Restriction (PORTO) study group, the definition of fetal growth restriction (FGR) in fetuses that weighed <10th percentile relied on the umbilical artery (UA) Doppler image only, which is a concept that was later reinforced by evaluation of the risk of adverse outcome in cases of normal umbilical Doppler images as residual. However, in an apparent paradox cerebroplacental ratio (CPR) was recently presented by the authors as a determinant parameter in the diagnosis of FGR.

Although we acknowledge the unquestionable importance of the PORTO study and applaud its large contribution for the current understanding of FGR, the relative importance of CPR in the FGR scenario merits further precision. Although the PORTO study lumps all FGR cases together, FGR is far from being a single disorder. Rather, it includes 2 different diseases with different causes: early-onset FGR is an infrequent and severe condition with hemodynamics pivoting on umbilical Doppler imaging, a circumstance that suggests a clear placental pathologic basis. On the other extreme, late-onset FGR is more frequent and milder, with subtle pathologic placental findings and hemodynamics that rely on brain-sparing (CPR), which suggests under-perfusion probably because of an imbalance between fetal needs and placental supply. In the PORTO study, 9 and 27 fetuses, respectively, with normal UA Doppler imaging and abnormal CPR presented adverse outcome. This is interesting. However, to evaluate the role of CPR, it had been far more interesting to indicate how many of the abnormal UA Doppler fetuses presented an abnormal CPR. Because the combination of normal UA Doppler plus abnormal CPR is characteristic of late-onset FGR, this might help to unravel the true composition of the PORTO sample. Although stratified data according to gestational age are missing, we suspect that the apparent predominance of abnormal UA Doppler imaging (n = 418) over abnormal CPR (n = 146) is reflecting a biased trend to early onset FGR in the PORTO sample. If, despite the mixed-data composition, CPR is still able to improve the ability of early UA Doppler imaging in the prediction of adverse outcome, we should expect a progressive improvement of CPR performance until term, when CPR associates with acid base status even better than fetal biometry.