The objective of this study was to determine the 10th, 50th and 90th percentiles of birthweight, by gestational age and sex, for newborns covered by the Mexican Institute of Social Security (IMSS) in the State of Chihuahua. To generate the database, we used IMSS hospitals' records in the State of Chihuahua, covering the period between 1 January 2000 and 31 December 2004. We included singleton live births only, and excluded babies with congenital malformations. The birthweights of 88,368 children born at 21-44 weeks of gestation comprised our data. From these data, we calculated the 10th, 50th and 90th percentiles for each sex, at 32-44 weeks of gestation. The observed cutoffs for the 10th percentile in our population were 40-250 g higher than those reported in other references with Mexican populations. These results constitute an updated birthweight reference that will allow the identification of newborns in the North region of the country with low birthweight-for-gestational age. Such information can be a useful instrument for preventing or diminishing associated risks.