The effect of high altitude and other risk factors on birthweight: Independent or interactive effects?

Objectives. This study examined whether the decline in birthweight with increasing altitude is due to an independent effect of altitude or an exacerbation of other risk factors. Methods. Maternal, paternal, and infant characteristics were obtained from 3836 Colorado birth certificates from 1989 through 1991. Average altitude of residence for each county was determined. Results. None of the characteristics related to birthweight (gestational age, maternal weight gain, parity, smoking, prenatal care visits, hypertension, previous small-forgestational-age infant, female newborn) interacted with the effect of altitude. Birthweight declined an average of 102 g per 3300 ft (1000 m) elevation when the other characteristics were taken into account, increasing the percentage of low birthweight by 54% from the lowest to the highest elevations in Colorado. Conclusions. High altitude acts independently from other factors to reduce birthweight and accounts for Colorado's high rate of low birthweight.