Why do they measure the MCA in maternal fetal ultrasound?

Problem with baby. Blood flow in the MCA (middle cerebral artery) may be abnormal if a baby is not getting enough oxygen, either because it has anemia so that it's blood cannot carry enough oxygen, or because it is not getting enough oxygen from the placenta. If your baby is not growing well, or if you have an infection or problem that might be causing anemia, your doctor may look at the baby's MCA with ultrasound.
Fetal anemia/growth. Drs. Carpenter/deter/mari described the use of the middle cerebral artery peak systoloc velocity in 2000 as an accurate non-invasive method for assessing fetal anemia. Dr. Baschat has more recently described the use of the MCA systolic/diadtolic ratio as a non-invasive marker for near-term feral growth restriction; it is decreased when the fetal brain-sparing reflex is activated.