The objective of this study is to establish differentials in birth weight (BW) and related factors, in term newborns (NB) of Spanish (SP) and Colombian (CO) immigrant mothers living in Spain, between 2001-2005. Data on the NB population of SP and CO mothers was retrieved from the National Statistical Bulletin of Birth in Spain. We analysed the association with BW (Low birth weight -LBW- insufficient weight -IW- macrosomia), by the nationality of the mother; taking into account variables such as the intergenesic interval, maternal age, number of live children, maternal occupation and sex of NB. The analysis was based on frequencies and the estimation of simple and adjusted odds ratios (OR) by means of logistic regression with 95% confidence intervals (95% CI). A higher prevalence of LBW was found in SP mothers (3.4%) than in their CO counterparts (2.1%). In SP mothers a higher risk of LBW (aOR 1.89, 950% CI 1.65- 2.16) and IW (aOR 1.49, 95% CI 1.51- 1.57) was observed. In CO mothers a higher percentage of macrosomia was found (8.0%). Also, a higher percentage of LBW was observed in female new borns (SP4.1%; CO 2.7%) as well as IW (PI (SP 25.6%; CO 19.6%) (p < 0.001). Mothers aged > 40 years and having 4 or more children were associated with LBW in both nationalities. As a conclusion, NB of Colombian mothers presented a lower prevalence of LBW and IW, which could be explained by the healthy migrant effect.