Birth weight and bone mineral density at 18–19 years: birth cohort 1997–1998

OBJECTIVE: To analyze the association between birth weight and bone mineral density (BMD) in adolescence. METHODS: A birth cohort study in São Luís, Maranhão, using data from two moments: at birth and at 18–19 years. Exposure was the birth weight in grams, continuously analyzed. The outcome was BMD, using the Z-score index (whole body) measured by double X-ray densitometry (Dexa). A theoretical model was constructed in acyclic graphs to identify the minimum set of adjustment variables – household income, the mother knowing how to read and write at the time of birth, prenatal care, tobacco use during pregnancy, and parity — to evaluate the association between birth weight and bone mineral density in adolescence. Multiple linear regression was used in Stata 14.0 software. A 5% significance level was adopted. RESULTS: From 2,112 adolescents, 8.2% had low birth weight and 2.8% had a low BMD for their age. The mean full-body Z-score was 0.19 (± 1.00). The highest birth weight was directly and linearly associated with BMD values in adolescence (Coef.: 0.10; 95%CI: 0.02–0.18), even after adjustment for the variables household income (Coef.: -0.33; 95%CI: -0.66–0.33) and the mother knowing how to read and write (Coef.: 0.23%; 95%CI: 0.03–0.43). CONCLUSION: Although after adjusting the variables the association attenuated, birth weight positively and linearly relates to BMD in adolescence.