Figure 2 Forced expiratory volume in 1 second (FEV₁) in litres in (A) men and (B) women at age 31 years by tertile of birth weight stratified by adult characteristics of men and women in the 1966 Northern Finland Birth Cohort study. The figure shows mean values and 95% confidence limits. Values obtained from regression models and adjusted for gestational age at birth, maternal smoking during pregnancy and factors at 31 years: height, weight, cigarette smoking (never, former and current) and physical activity level (high versus low) except for factor under consideration or weight for analysis using body mass index (weight/height²). Smokers were current smokers and non‐smokers were never and former smokers combined. High physical activity level defined as participation in brisk activity of at least 20 minutes twice a week and low level if participation was less in duration and/or frequency per week. p for interactions: birth weight and smoking (men = 0.03, women = 0.31), birth weight and physical activity (men = 0.09, women = 0.41), and birth weight and body mass index (men = 0.54, women = 0.30).