The influence of gestational age and smoking habits on the risk of subsequent preterm deliveries

Background Previous preterm delivery and maternal smoking are associated wi th increased risks of preterm delivery. It is not known whether gestational age at the time of a preterm delivery is correlated with gestational age i n successive preterm deliveries and whether changes in smoking habits influ ence the subsequent risk of preterm delivery. Methods We studied the associations among smoking habits, previous very pre term or moderately preterm delivery (before 32 weeks and at 32 to 36 weeks, respectively), and the risk of a subsequent very preterm or moderately pre term delivery in a population-based cohort of 243,858 women in Sweden betwe en 1983 and 1993. The results were adjusted for covariates known to be asso ciated with preterm delivery. Results The odds ratios for very or moderately preterm delivery in a subseq uent pregnancy among women with a previous very preterm delivery, as compar ed with women who had a previous term delivery, were 12.4 (95 percent confi dence interval, 9.1 to 17.0) and 7.1 (95 percent confidence interval, 6.0 t o 8.4), respectively. Among women with a previous moderately preterm delive ry, the corresponding odds ratios were 2.3 (95 percent confidence interval, 1.9 to 3.0) and 5.9 (95 percent confidence interval, 5.5 to 6.3), respecti vely. The odds ratios for a very preterm second delivery among the women wh o smoked 1 to 9 cigarettes per day and those who smoked 10 or more cigarett es per day, as compared with nonsmokers, were 1.4 (95 percent confidence in terval, 1.1 to 1.7) and 1.6 (95 percent confidence interval, 1.3 to 2.0), r espectively. The corresponding odds ratios for moderate preterm delivery we re 1.3 (95 percent confidence interval, 1.2 to 1.4) and 1.5 (95 percent con fidence interval, 1.4 to 1.6). The women who quit smoking between pregnanci es were not at increased risk for very or moderately preterm delivery, wher eas the women who started to smoke in the second pregnancy had the same ris k as those who continued to smoke. Conclusions The risk of a very preterm delivery in successive pregnancies i s increased primarily among women with a previous very preterm delivery. Ch anges in smoking habits influence the risk of preterm delivery as well. (N Engl J Med 1999;341: 943-8.) (C) 1999, Massachusetts Medical Society.