The fetal head sparing theory is one that underpins asymmetrical intra-uterine growth restriction, where the difference between normal head circumference and decreased abdominal circumference is attributed to the fetus's ability to preferentially supply the cerebral, coronary, adrenal and splenic circulations.
In a situation of chronic fetal hypoxaemia, the fetus redistributes its cardiac output to maximize the oxygen supply to brain by vasodilation of the cerebral arteries thereby causing a decrease in the left ventricular afterload.
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