Maternal oxygen delivery is not related to altitude- and ancestry-associated differences in human fetal growth

Fetal growth is reduced at high altitude, but the decrease is less among long-resident populations. We hypothesized that greater maternal uteroplacental O₂ delivery would explain increased fetal growth in Andean natives versus European migrants to high altitude. O₂ delivery was measured with ultrasound, Doppler and haematological techniques. Participants (n = 180) were pregnant women of self-professed European or Andean ancestry living at 3600 m or 400 m in Bolivia. Ancestry was quantified using ancestry-informative single nucleotide polymorphims. The altitude-associated decrement in birth weight was 418 g in European versus 236 g in Andean women (P < 0.005). Altitude was associated with decreased uterine artery diameter, volumetric blood flow and O₂ delivery regardless of ancestry. But the hypothesis was rejected as O₂ delivery was similar between ancestry groups at their respective altitudes of residence. Instead, Andean neonates were larger and heavier per unit of O₂ delivery, regardless of altitude (P < 0.001). European admixture among Andeans was negatively correlated with birth weight at both altitudes (P < 0.01), but admixture was not related to any of the O₂ transport variables. Genetically mediated differences in maternal O₂ delivery are thus unlikely to explain the Andean advantage in fetal growth. Of the other independent variables, only placental weight and gestational age explained significant variation in birth weight. Thus greater placental efficiency in O₂ and nutrient transport, and/or greater fetal efficiency in substrate utilization may contribute to ancestry- and altitude-related differences in fetal growth. Uterine artery O₂ delivery in these pregnancies was 99 ± 3 ml min^-1, ∼5-fold greater than near-term fetal O₂ consumption. Deficits in maternal O₂ transport in third trimester normal pregnancy are unlikely to be causally associated with variation in fetal growth.