Antenatal surveillance

The middle cerebral artery (MCA) is a branch of the circle of Willis in the base of the skull

A transverse section of the fetal head is obtained and color flow mapping is used to identify the circle of Willis and the MCA. The pulsed wave Doppler gate is then placed on the proximal one-third of the MCA. The angle of insonation should be zero degrees. Attention should be taken to avoid any unnecessary pressure on the fetal head because this increases both the peak systolic velocity (PSV) and pulsatility index (PI).

• In normal pregnancies the PSV increases and the PI decreases with gestation
• Decreased PI is observed in fetal hypoxemia and fetal hypertension
• Increased PSV is observed in fetal anemia and fetal hypercapnia (increased paCO2)