Cerebroplacental ratio

Cerebroplacental ratio (CPR) is an obstetric ultrasound tool used as a predictor of adverse pregnancy outcome in both small for gestational age (SGA) and appropriate for gestational age (AGA) fetuses. An abnormal CPR reflects redistribution of cardiac output to the cerebral circulation, and has been associated with intrapartum fetal distress, increased rates of emergency cesarean and NICU admissions and poorer neurological outcomes. 

Measurement

It is calculated by dividing the Doppler pulsatility index of the middle cerebral artery (MCA) by the umbilical artery (UA) pulsatility index:

CPR = MCA PI / UA PI

The index will reflect mild increase in placental resistance with mild reductions in the fetal brain vascular resistance. An abnormal cerebroplacental ratio may result in the following conditions:

• low normal range MCA and upper normal range UA PI
• abnormal low MCA and normal UA PI
• abnormal low MCA and high UA PI

It follows then that detection of perinatal risk based on CPR may occur in the setting of reassuring UA PI (but abnormal MCA), or even if both UA and MCA PI are within the normal range. 

Interpretation

Various studies have variably defined the threshold of abnormal CPR as either ratio <1 ^ref, ratio <1.08^ref, in terms of MoM ^ref or based on centiles.  The only longitudinal study known to the article editor at the time of writing defines abnormal CPR as <5^th centile ².