The fetus develops normally in a hypoxic environment but exaggerated hypoxia late in pregnancy is a worrisome sign often observed in hypertensive disorders of pregnancy, placental insufficiency, or fetal growth restriction (FGR). Serial fetal biometry and the cerebroplacental ratio (CPR, calculated as the middle cerebral artery [MCA] / the umbilical artery [UmbA] pulsatility indices [PI]), are commonly used to indicate fetal "brain sparing" resulting from exaggerated fetal hypoxia. But unclear is the extent to which a low CPR indicates pathology or is a physiological response for maintaining cerebral blood flow. We studied 31 appropriate for gestational age (AGA) pregnancies at low (LA, 1670 m) or high (HA, 2879 m) altitude, given the chronic hypoxia imposed by HA residence, and 54 LA women with a clinical diagnosis of FGR. At week 34, the MCA PI was lower in the LA-FGR than the LA-AGA group but lower still in the HA-AGA compared to either LA groups due to a trend toward higher end-diastolic velocity (EDV). We concluded that the lower MCA PI was likely due to greater cerebral vasodilation in the HA AGA group and an indication of physiological versus pathological fetal hypoxia. Future reporting of serial MCA and UmbA values and their determinants along with the CPR could improve our ability to distinguish between physiological and pathological fetal brain sparing. Keywords: Birth weight, Cerebroplacental ratio, Fetal physiology, HDP, High altitude.

Department of Obstetrics and Gynecology University of Colorado Denver Anschutz Medical Campus Aurora Colorado USA

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