Background The intrauterine period has been considered as a very sensitive period in which nutritional and/or hormones changes appear to play an essential role in the subsequent control of body weight.

Objectives Classify the nutritional status of children, analysing its relationship with birth weight.

Methods Cross-sectional and observational study comprising 792 preschool children, average age 4.39 years old (SD = 0.911), residents in a centre region of Portugal. Children’s anthropometric measurement was obtained and the classification was based on the NCHS reference (CDC, 2000) and the birth weight classification on the WHO (2001).

Results Globally 66% had normal weight, 31.3% were overweight (including 12.4% obesity) and 2.7% low-weight but the differences shown to be independent from age and gender of children. 91.9% of girls and 87.1% of boys was born with appropriate weight for gestational age while 6.5% and 3.9% were born respectively lightweight and large for gestational age.

The association between birth weight and overweight revealed that 7.4% of children with overweight were born large. The relationship was statistically significant (χ² = 21.130, p = 0.002), implying that a higher birth weight was associated with increased risk of overweight in childhood with a probability greater than 8 times (OR = 8.486, 95% CI = 2.443 to 29.483) (χ² = 13.636, p = 0.000).

Conclusion The results suggest significant effect of birth weight on the development of later overweight. So, children born with high weight require further monitoring and promotion of an adequate dietary pattern, in order to control early its nutritional status.