A quasi-experimental analysis of maternal altitude exposure and infant birth weight

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.