Maternal hemoglobin level and fetal outcome at low and high altitudes

Both, low (<7 g/dl) and high (>14.5 g/dl), maternal hemoglobin (Hb) levels have been related to poor fetal outcome. Most studies have been done at low altitude (LA). Here, we have sought to determine whether this relationship exists at both high and low altitude, and also whether there is an adverse effect of high altitude (HA) on fetal outcome independent of level of maternal hemoglobin. The study is based on a retrospective multicenter analysis of 35,449 pregnancies at LA and six other cities above 3000 meters. In analyses of all women at both LA and HA, those with Hb <9 g/dl had odds ratios (ORs) and 95% confidence intervals (CI) of 4.4 (CI: 2.8-6.7), 2.5 (CI: 1.9 -3.2), and 1.4 (CI: 1.1-1.9) for stillbirths, preterm, and small for gestational age (SGA) births, respectively, compared with women with 11-12.9 g/dl of Hb, after adjustment for confounders. These risks by hemoglobin level differed little between women at LA and HA, suggesting that no correction of the definition of anemia is necessary for women at HA. Women living at high altitude with hemoglobin >15.5 g/dl had higher risks for stillbirths (OR: 1.3; CI: 1.05-1.3), preterm (OR: 1.5; CI 1.3-1.8), and SGA births (OR: 2.1, CI 1.8 -2.3). There was also a significant adverse effect of living at HA, independent of hemoglobin level for all three outcomes (OR: 3.9, 1.7, and 2.3; CI: 2.8 -5.2, 1.5-1.9, and 2.1-2.5) for stillbirths, preterms, and SGA respectively, after adjusting for hemoglobin level. Both, high and low maternal hemoglobin levels were related to poor pregnancy outcome, with similar effect of low hemoglobin in both LA and HA. Our data suggest, that maternal hemoglobin above 11 g/dl but below 13 g/dl is the area of minimal risk of poor adverse outcomes. Living at HA had an adverse effect independent of hemoglobin level.