Objectives: The purpose of our study was to investigate the combined interactive effects of maternal age, parity, and smoking on pregnancy outcome.
Study design: This was a population-based Swedish study (n = 538,829).
Results: Multiple logistic regression analysis showed that the smoking-related effect on the relative increase in the odds ratio of low birth weight and preterm delivery was significantly greater among multiparous patients than nulliparous; among multiparas, smoking increased the odds ratios for low birth weight and preterm delivery by 2.4 and 1.6; the corresponding relative increases in the odds ratios among nulliparas were 1.7 and 1.1, respectively. With advancing maternal age there was a smoking-related relative increase in the odds ratios for small-for-gestational-age births. Moreover, the age effect on the relative increase of low birth weight, preterm delivery, and small-for-gestational-age births was greater among nulliparas than multiparas.
Conclusions: Older smokers are at an especially high risk for small-for-gestational-age births, and parous smokers are at an especially high risk for low birth weight and preterm delivery.