摘要：Objectives. We analyzed singleton births to determine the relationship between birth weight and altitude exposure. Methods. We analyzed 715 213 singleton births across 74 counties from the western states of Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, and Washington from January 1, 2000, to December 31, 2000. Birth data were obtained from the Division of Vital Statistics, National Center for Health Statistics, for registered births. Results. Regression analyses supported previous research by showing that a 1000-meter increase in maternal altitude exposure in pregnancy was associated with a 75.9-gram reduction in birth weight (95% confidence interval = −84.1, −67.6). Quantile regression models indicated significant and near-uniform depressant effects from altitude exposure across the conditional distribution of birth weight. Bivariate sample-selection models showed that a 1000-meter increase in altitude exposure, over and above baseline residential altitude, decreased birth weight by an additional 58.8 grams (95% confidence interval = −98.4, −19.2). Conclusions. Because of calculable health care–related costs associated with lower birth weight, our reported results might be of interest to clinicians practicing at higher altitudes. Research shows that infant birth weight appears to significantly decline at higher altitude. 1 At least 2 biologically plausible explanations underlie the birth weight and altitude association, assuming the association is causal. One explanation is that maternal oxygen deprivation is involved. 2–4 Because the driving pressure for gas exchange in lungs decreases at higher altitude, lower oxygen per breath of air by pregnant mothers may induce growth-limiting hypobaric hypoxia. Others question the hypoxia pathway, noting that sufficient oxygen delivery to the uterus is achieved by hematologic and metabolic adjustments in high-altitude conditions. 5,6 More recently, researchers noted the possibility of a second biological mechanism that involves a glucose pathway. Zamudio et al 6 found that umbilical venous and arterial glucose concentrations were lower at high altitude, resulting in lower glucose delivery to and consumption by the fetus. Anaerobic consumption of glucose by the placenta at high altitudes appears to reduce glucose availability to the fetus. Hypoglycemia may therefore also explain lower birth weights at higher altitudes. 5,6 Whatever the precise biological mechanisms involved, empirical studies reporting a negative association between altitude and infant birth weight 1,5,7–9 are questionable on 3 grounds. First, previous studies of birth weight typically analyzed smaller populations at only a handful of conveniently selected altitude locations. 9–12 With a few exceptions, 13–16 the depressant effects of altitude across a wide range of altitude positions and over a large number of birth events are unknown. Technically, many existing studies have excessive off-support inference where altitude effect estimates are pure extrapolation or off-support of the data. 17 Second, it is unclear whether the depressant effect of altitude exposure operates uniformly or differently across the conditional distribution of infant birth weight. 16 Third, and perhaps most importantly, households are known to self-select into higher altitude locations. In the United States, topographic variation, mountain vistas, and higher altitude settings are known amenity pull factors in migration decisions by households. 18–20 Because of residential self-selection, maternal altitude exposure is a nonrandomly assigned treatment effect. In this study, we addressed these 3 research design issues and arrived at a more precise quantification of the depressant effects of altitude on fetal birth weight. We did so by analyzing data on 715 213 registered births across 74 counties in the Western United States in 2000. First, infant birth weight was estimated with least-squares regression to determine mean effects of altitude. We then used nonparametric regression to determine quantile effects of altitude across the conditional distribution of birth weight. Third, to causally identify the depressant effects of altitude on birth weight, we exploited information on random altitude movements of mothers during pregnancy. We present evidence showing that up-moving and down-moving mothers were statistically similar with respect to demographic characteristic composition and conditional likelihoods of both moving and up-moving in altitude during pregnancy. We used an econometric selection model to account statistically for possible correlation between mobile mothers and underlying fetal health. We conclude the article with a brief consideration of the implications of results for clinical practice at a high altitude, and how the observed depressant effects of altitude might shed light on the widely reported recent decline in birth weight in the United States.
