Low Cerebroplacental Ratio

A low cerebroplacental ratio is a ratio of the middle cerebral artery pulsatility index to the umbilical artery pulsatility index below the 5th centile or below 1.08 at term. It signals redistribution of blood flow to the brain and subtle placental dysfunction. It matters because it predicts intrapartum fetal distress, emergency caesarean section and stillbirth, even in fetuses of normal size.

• Can be present in normally grown term fetuses.
• Risk factors: smoking, hypertension, preeclampsia, diabetes, post term pregnancy, maternal age above 35 years.
• It prompts closer surveillance and earlier delivery.

Women are usually well and unaware. Some report reduced fetal movements, and the fundal height may be normal or small.

• Normal or small abdominal circumference.
• Reduced movements in some.
• Differential diagnosis: normal variation, growth restriction, technical error, fetal head compression by the probe, fetal anaemia, post term placental ageing.
• Complications: stillbirth, intrapartum hypoxia, emergency caesarean section, neonatal acidosis, neonatal intensive care admission.

The approach is a Doppler measurement at term or when growth or movements are concerning.

• Middle cerebral artery and umbilical artery pulsatility indexes at one visit, during fetal rest and with an angle near 0 degrees; ratio below 1.08 or below the 5th centile is abnormal.
• Estimated fetal weight, abdominal circumference, deepest pocket, uterine artery Doppler.
• Cardiotocography; maternal blood pressure, urine protein, glucose.
• Repeat in 3 to 7 days to confirm a persistent low ratio.

The principle is delivery by induction at 37 to 38 weeks with continuous fetal monitoring in labour.

• Supportive: daily fetal movement counts; left lateral rest; stop smoking; deliver in a facility with continuous cardiotocography and caesarean ability.
• Pharmacological: misoprostol 25 micrograms vaginal 6-hourly (maximum 4 doses) for induction with an unscarred uterus; oxytocin starting at 2 mU/minute and doubling every 30 minutes to a maximum of 32 mU/minute; labetalol 200 mg oral every 8 to 12 hours (maximum 2.4 g daily) or nifedipine modified release 30 to 60 mg oral once daily (maximum 120 mg) for hypertension; oxytocin 10 IU IM at birth; betamethasone 12 mg IM twice, 24 hours apart, only if delivery is needed before 35 weeks.
• Procedures: induction at 37 weeks when the fetus is small, and at 38 weeks when normally grown; Foley catheter with a 30 mL balloon if the uterus is scarred; continuous cardiotocography in labour; caesarean section for pathological tracing; earlier delivery for absent or reversed end diastolic flow or reduced movements.
• Special groups: diabetes in pregnancy with a low ratio needs induction at 37 to 38 weeks with insulin infusion in labour of 1 to 2 units per hour and hourly glucose.
• Red flags: reduced fetal movements, abnormal umbilical artery flow, oligohydramnios, pathological cardiotocography, bleeding.
• Follow-up: repeat Doppler and cardiotocography twice weekly until delivery; newborn glucose check at 2 hours, with glucose below 2.6 mmol/L treated with dextrose 10 percent 2 mL/kg IV over 10 minutes then 80 mL/kg per day.