Hemoglobin, altitude and birth weight: does maternal anemia during pregnancy influence fetal growth?

OBJECTIVE: To investigate the relationship between maternal hemoglobin concentration, altitude and birth weight.

STUDY DESIGN: Birth weights in 235 term pregnancies were investigated for their dependence on maternal hemoglobin concentration after other maternal and pregnancy-specific influences on fetal weight were taken into account. The additional predictive value of hemoglobin concentration on birth weight was assessed using multiple regression. Using published data, the relationship of hemoglobin concentration to altitude was determined, as was the effect of increasing altitude on birth weight. The quantitative effect of hemoglobin concentration on birth weight was correlated with the effect of altitude on hemoglobin concentration to assess whether this could account for the known decrease in birth weight with increasing altitude.

RESULTS: Birth weights ranged from 2,220 to 4,850 g (mean, 3,505+/-443), and hemoglobin concentrations ranged from 9.3 to 13.5 g/dL (mean, 11.6+/-0.8). Apart from other known predictive variables, the variation in maternal hemoglobin concentrations at constant altitude independently explained 2.6% of the variance in birth weight (r=-.18, P=.003). Term birth weight was reduced by 89 g for each 1.0 g/dL increase in hemoglobin concentration (P<.01). For every 1,000-m increase in altitude, hemoglobin concentration increased by 1.52 g/dL and birth weight decreased by 117 g.

CONCLUSION: Birth weight correlates negatively with maternal hemoglobin concentration. This is consistent with the well-known effect of high-altitude exposure during pregnancy, which increases both hematocrit and blood viscosity and lowers birth weight. The quantitative effect on birth weight of increasing maternal hemoglobin concentration at constant altitude is within 13% of the change in birth weight that can be attributed to the change in hemoglobin concentration associated with increases in altitude.