Introduction. A fetus that does not reach the expected growth potential in utero is considered small for gestational age (SGA). Such restriction depends on genetic and/or environmental factors, being altitude a very relevant factor. This study analyzes the spatial distribution of the prevalence of SGA and its secular trend in Jujuy (1991–2014). Materials and methods. The records of 308 469 live births in Jujuy (Health Statistics and Information Department) were analyzed. The prevalence of SGA (weight/gestational age < P10 and < P3) was estimated for sex according to the INTERGROWTH-21 st standard in the ecoregions of Jujuy (Valle and Ramal –less than 2000 MASL–, Puna, and Quebrada) across 3 periods (1991–2000, 2001–2009, 2010–2014) and proportions were compared. The secular trend was assessed using the Joinpoint regression analysis. Results. The overall prevalence of SGA was 2.3% (< P3) and 7% (< P10). Significantly higher values were observed in Puna and Quebrada in both SGA categories and across all periods. Only in Valle, significant differences were observed between sexes across all periods. The prevalence of SGA showed a significant downward secular trend at a provincial and regional level, and this was greater in Quebrada (5.2% < P3 and 3.5% < P10). Conclusions. A consistent and significant decrease in the prevalence of SGA has been observed since the 1990s in Jujuy, where altitude is itself a determining factor of size at birth, since the Puna and Quebrada regions showed the highest prevalence of SGA during the entire period.