Scaling placental oxygen diffusion to birthweight: studies on placentae from low- and high-altitude pregnancies.

Stereological analysis of tissue sections has been used to estimate partial and total conductances of the human placental oxygen diffusion pathway. Analyses were undertaken for neonates and term placentae from populations living at low (400 m) and high (3600 m) altitude in Bolivia. Relationships between conductances and birthweight (and between trophoblast volume and birthweight) were explored by linear regression analysis and allometry. At both altitudes, the three partial conductances on the maternal side of the oxygen pathway scaled to birthweight in a similar fashion. This similarity did not extend to conductances on the fetal side. Beyond a limiting weight of roughly 3 kg, the highland fetus was disadvantaged in terms of its placental oxygen diffusive conductances and trophoblast volume. The findings support the notion that the structural adaptations seen at high altitude are more successful on the maternal rather than the fetal side of the placenta.