The strong association of very low birth weight (VLBW; <1500 g) with infant mortality is well known. There are no data related to VLBW trends in Brazil although findings from some large surveys carried out in small- and medium-sized cities have demonstrated an increase in low birth weight (LBW; <2500 g) rates over the last 30 years. The aim of this study was to evaluate the secular trend of VLBW in the city of Porto Alegre, a large city in a developed area in southern Brazil, and the potential determinants of this trend during the 1990s and early 2000s. This is a registry-based study. Data were obtained from birth certificates of all live births in the city from 1994 to 2005. The variables analysed were: VLBW as the dependent variable and maternal age and schooling, type of delivery, type of hospital, number of live births, gestational age, newborn gender and unemployment rate. The incidence ratio rate (IRR) using Poisson regression was calculated to identify possible trends in VLBW rates. Poisson regressions were performed in order to assess the influence of some independent variables on VLBW. A total of 257,740 singleton newborns were delivered in the city during the period, with a steady reduction in the total number of live births per year from 23,296 in 1994 to 18,325 in 2005. The results showed a small but significant increase in VLBW (p for trend=0.049). There was a significant trend towards adequacy for gestational age per birth weight, suggesting a reduction in rates of intrauterine growth restriction (IUGR) (p<0.001). The findings showed a significant increase of 1% per year in the probability of VLBW. The main risk factors were related to VLBW mothers with low levels of schooling, public hospitals and multiparity and nulliparity. After adjustment, nulliparity remained as a significant risk factor. The interaction between type of hospital and type of delivery indicated that the probability for VLBW was 3.6 times higher (p<0.001) among those born in public hospitals by Caesarean section than those born in private hospitals by vaginal delivery. The results show that southern Brazil is going through a demographic transition characterized by a significant decrease in number of live births associated with a small increase in VLBW rates, a decrease in IUGR rates and stabilization of LBW.

This study investigates the possible effects of pre-term births and low birth weight on infant mortality rates (IMRs) over a 15-year period in Ribeirão Preto, Brazil, based on surveys carried out in 1978/79 and 1994. The 1978/79 survey included 6750 births over a 12-month period and the 1994 survey 2846 births over a 4-month period. Infant deaths were retrieved monthly from the city register. Infant mortality rate decreased from 36·6 to 16·9 deaths per 1000 over 15 years. The decrease in IMR was larger in the 2500–2999 g group than in any other group. The observed falls in IMR were attributable to decreases in birth-weight-specific mortality rates. Likewise, there was a general decrease in IMR in mild, moderate and severe pre-term births. The incidence rate ratio of infant mortality between surveys was 0·46 (95% CI 0·34–0·63); it increased to 0·57 (95% CI 0·35–0·75) when adjusted for birth weight and other factors in the model and rose to 0·69 (95% CI 0·49–0·97) when adjusted for length of gestation and other variables. The increase in pre-term births and low birth weight may have had, at most, a marginal effect on the IMR. Progress in the care of newborns may have decreased the mortality risk, but even mild pre-term birth still has an impact on infant mortality. There is room for further improvement in IMR by tackling the high rates of pre-term birth.