Ertl R, et al. - Via extracting data on infant birth weights of the first and second pregnancies from same women from all Austrian birth certificates between 1984 and 2016, researchers examined the impact of a change of altitude of maternal living address on infant birth weight. There were identified and analyzed a total of 544,624 pair pregnancies. Among women with first birth at low altitude (200 m), the estimated second mean birth weight for those who remained at low altitudes was 3567 g and reduced to 3536 g for those who ascended (1200 m). On the other hand, among women with first births at high altitudes, the estimated birth weight of second birth at high altitude was 3414 g but increased to 3499 g vs those descended to lower altitudes. A statistically significant interaction was observed between altitudes of two births and birth weight. In the same mother from the first and second birth, a longitudinal negative effect of altitude on birth weight was demonstrated. It is likely that this association will be casual. Mothers' relocation to low-to-medium altitude may have profound effects on the birth weight of infants.