Part of this increase in LBW rate may be explained by a simultaneous increase in the multiple birth rate and in the number of newborns (NB) weighing 500 to 999 g and by a reduction in the stillbirth rate .
Studies monitoring LBW trends as well as changes in its risk factors over time are important in order to determine whether such factors are changing, thus allowing identification of targets for public health interventions.
Increasing LBW rate was also observed in Brazilian birth cohorts performed in Ribeirão Preto and Pelotas.
The rise in LBW rate paralleled reduction in the stillbirth rate, suggesting improvement in obstetrical and newborn care.
Maternal, socioeconomic and demographic factors associated with LBW differed between the two cohorts, except for smoking during pregnancy and parity that were significantly associated with LBW in both cohorts. LBW is due either to preterm birth, most common in developed countries, to intrauterine growth restriction (IUGR), more frequent in developing countries or to a combination of both.
There was an increase in maternal age ≥35 years and schooling ≥12 years.
The variables associated with LBW in 1997/98 were young maternal age ( Although LBW rate did not differ between the two cohorts, this apparent stability masked an increase up to 2001 and a decrease thereafter.
LBW rate is estimated to be 15% for developing countries and 7% for developed countries .
In Brazil, LBW rate increased significantly in its state capital cities from 8.5% in 1996 to 9.1% in 2010.
Factors associated with LBW are low maternal weight and height, multiple births, low calorie intake, hypertension during pregnancy, maternal smoking, genetic syndromes, hard maternal physical work, maternal exposure to toxic substances, and inadequate prenatal care use .
LBW rates differ in various regions of the world, being higher in less developed countries, since they are associated with unfavorable socioeconomic conditions.