Introduction
Predicting which pregnancies are at risk of developing adverse perinatal outcome, including fetal distress during labour, remains one of the major challenges obstetricians face. Much research has been devoted to developing strategies to identify these at‐risk pregnancies. The cerebro‐placental ratio (CPR) is one such measure that has been described as a predictor of adverse perinatal outcome.1 Although systematic and expert reviews of the available literature have shown CPR to be a good predictor of fetal distress in labour, as well as low arterial pH, neonatal intensive care unit (NICU) admissions and poor neonatal morbidity and mortality, it is not yet in regular clinical use.2, 3, 4
Calculation of the CPR
Cerebro‐placental ratio is the ratio that quantifies the brain‐sparing effects seen in placental insufficiency. It is calculated by dividing the middle cerebral artery (MCA) Doppler flow by the umbilical artery (UA) Doppler flow. The systolic/diastolic (SD) ratio, the resistance index (RI) and the pulsatility index (PI) have been used to calculate CPR; however, in recent studies, the PI was the preferred method. The largest cohort to date of CPR in an intrauterine growth‐restricted (IUGR) population, the PORTO study, showed that, irrespective of the CPR calculation used, brain sparing is significantly associated with an adverse perinatal outcome in IUGR.5
Pathophysiology
It was initially hypothesised that the UA Doppler is the first finding to change in placental insufficiency, followed by vasodilatation of the MCA.6 Therefore, in standard practice, the MCA Doppler is not measured if the UA Doppler is within normal limits. However, recent data from Unterscheider et al. showed, in a large cohort of IUGR fetuses, the UA Doppler was the first to become abnormal in only 46% of cases.7 Another study from 2011 showed that, in a late IUGR cohort, CPR is a better predictor than MCA alone, with the UA Doppler being mainly normal in this group.8 More recently CPR was noted, in appropriately grown fetuses, to be as important a marker of low neonatal pH as is being small for gestational age (SGA).9
Data interpretation
The data available is heterogeneous with regard to defining abnormal CPR. Different cut‐offs, either centiles, values under 1, z‐scores or multiples of the mean, have been used. When comparing different thresholds, Bligh et al. showed CPR less than the 10th centile had the best overall test performance in low‐risk pregnancies at term.10 Prior et al. showed that using a cut‐off of <0.6765 MoM yielded a similar positive predictive value for caesarean section because of presumed fetal compromise as using the 10th centile.11, 12
Timing the measurement
In deciding when to measure the CPR, the suggestion was made that its predictive value decreases over time.13 More recent studies agree that the sensitivity of CPR in cases with IUGR measured before 34 weeks was higher than in the overall population.14 Secondary analysis of the PORTO study also showed that an abnormal CPR value was best interpreted at less than 34 weeks gestation.15
CPR in restricted vs. appropriate growth
Most studies analysing the predictive value of CPR have been performed in early and late IUGR cohorts and show CPR to be a significant predictor of, for example, operative delivery for fetal distress.11, 13, 16, 17, 18 When evaluating CPR in appropriately grown fetuses, recent reports used data from healthy post‐term pregnancies. They found that CPR was a good predictor of neonatal outcome and recommended clinical application in a low‐risk cohort.11, 19, 20 Another recent report showed that a CPR below the 10th centile measured at 35–37 weeks gestation was associated with a greater risk of intrapartum compromise. The group with abnormal CPR was almost five times more likely to need an emergency delivery for fetal compromise than a spontaneous vaginal delivery.21
Conclusion
The value of CPR as a predictor of fetal distress within the growth‐restricted fetus population has been proven. Further research should now be aimed at investigating CPR in appropriately grown fetuses after 34 weeks, as part of a risk assessment for predicting fetal distress in labour. CPR in combination with the estimated fetal weight could be used to develop a risk score, to improve the identification of at‐risk fetuses throughout the antenatal period up until labour. To prevent heterogeneity of further research data, one cut‐off should be used and we propose using the 10th centile as that cut‐off. In the light of the current available data, CPR is an aid in predicting which fetuses are at risk of developing adverse perinatal outcome and it is time to start putting it to good use.
References
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