The Effect of Altitude on Birthweight/Length Ratio: A Population-Based Study Over 36 Years in an Altitude Range from Sea Level to 1,700 m.

Klebermass-Schrehof Katrin, Thomas Waldhoer, and Lin Yang. The effect of altitude on birthweight/length ratio: a population-based study over 36 years in an altitude range from sea level to 1,700 m. . 23:90-95, 2022. The negative effect of altitude on fetal growth has been documented, but it is unknown whether this effect changes over time. We investigated the effect of altitude on infant birthweight/length ratio as well as its potential dependence on gestational age and year of birth in the range from sea level up to 1,700 m (Austria). Data on maternal characteristics, infant birthweights, and infant lengths were extracted from all Austrian birth certificates between 1984 and 2019. A total of 2,240,439 birth certificates were identified and analyzed. The effect of altitude on birthweight/length ratio was -2.66 g/cm (95% confidence interval [CI]: -2.77 to -2. 54) per 1,000 m increased altitude in 1984-1986, which decreased to -1.96 g/cm (95% CI: -2.09 to -1.82) in 2017-2019. The effect of altitude on birthweight/length ratio remained constant for preterm infants, which fluctuated around -1.5 g/cm. For term infants, the negative effect of altitude on birthweight/length ratio attenuated from -3 to -1.9 g/cm over time with a stronger decrease for infants born between 41 and 42 compared with those between 37 and 40 weeks of gestation. In summary, our data demonstrate a strong effect of altitude on birthweight/length ratio over 36 years with a smaller effect in recent years and a stronger effect in infants born around term age compared with preterm infants.