Objectives To investigate pre-gestational risk factors for spontaneous preterm birth and, the role of smoking and its cumulative effects on prematurity.
Methods A case-control study analyzed all births (2,198) occurring in a tertiary maternity hospital between April 2002 and July 2004. Spontaneous preterm births of single and live newborns without malformations were selected as cases. Controls were all the term births of live and single newborns without malformations during the same period. Three outcomes were studied: all preterm births (< 37 weeks), less than 35 weeks and less than 32 weeks of gestational age. Logistic regression was used to obtain the independent effect of pre-gestational risk factors.
Results Maternal age of less than 20 years, low schooling, low maternal pre-gestational body mass index and smoking showed significant, independent association with spontaneous preterm birth for the three outcomes. For all these risk factors, excepting maternal smoking, odds ratios increased with decreasing gestational age at birth and this trend was significant for low maternal age and low pre-gestational body mass index.
Conclusions The cumulative effects of smoking calls for the need to encourage smoking cessation among pregnant women, especially those who are underweight and in the older age groups, because of the increased risk of delivering premature babies.