Maternal age and low birth weight: a reinterpretation of their association under a demographic transition in southern Brazil.

To evaluate the relationship between changes in fecundity rates and maternal age and the impact of maternal age on low birth weight (LBW) rates in a developed region in southern Brazil. A time series study evaluating birth weight and maternal ages through the born alive information system (SINASC) in Porto Alegre from 1996 to 2008. The Chi-square test for trends was used to evaluate the trend of LBW and fecundity rates at each maternal age. Population attributed risk (PAR) was used to calculate the impact of maternal age on LBW rates. The study included 271,100 newborns. There was a significant reduction in fecundity rates in all age groups younger than 34 years, but especially in the groups between 20 and 29 years. Overall LBW increased from 9.3 to 10.7 % (P < 0.001). The PAR for LBW showed a reduction in the group from 17 to 19 years (from 1.7 % in 1996-1999 to 0.1 % in 2004-2008), and an increase in the groups from 35 to 39 years (from 2.0 % in 1996-1999 to 2.3 % in 2004-2008) and above 40 (from 1.1 % in 1996-1999 to 1.5 % in 2004-2008). There was a significant change in fecundity pattern in the last 12 years in southern Brazil. Adolescent mothers were surpassed by mothers over 30 years of age in terms of vulnerability for LBW babies. The results show a change in the maternal age distribution towards older mothers, accompanied by an increasing incidence of LBW. This demographic transition also involved a paradoxical pattern with a remarkable reduction in fecundity rates in intermediate maternal age groups with concomitant increase in their risk for LBW.