We defined neonates as small, appropriate, or large for gestational age (SGA, AGA, LGA) based on birth weight, length, and head circumference. We analyzed the effects on the somatic classification of maternal body mass index (BMI) (<18.5, 18.5-24.99, 25.0-29.99, ≥ 30) and smoking during pregnancy (0, 1-7, 8-14, ≥ 15 cigarettes daily). Data were from the German Perinatal Survey (1998-2000; 433,669 cases). The following refers to the classification by birth weight. In the normal maternal weight population SGA rates increased with cigarette consumption: 9.8%, 17.8%, 21.6%, and 25.4% for non-smokers, and smokers of 1-7, 8-14, and ≥ 15 cigarettes daily, respectively. In non-smoking underweight women the SGA rate was 17.4%. In underweight smokers of ≥ 15 cigarettes daily the SGA rate was 38.5% [odds ratio 5.77, 95% confidence interval 5.10-6.53, compared with normal weight non-smokers]. In the normal maternal weight population, LGA rates were 9.9%, 5.3%, 4.6%, and 3.5% for non-smokers, and smokers of 1-7, 8-14, and ≥ 15 cigarettes daily, respectively. In the obese, LGA rates were 20.9% (non-smokers) and 11.4% (≥ 15 cigarettes). Similar findings were obtained for the somatic classifications based on birth length and head circumference. Results for the various combinations of maternal BMI and smoking status in the three classification systems are described. Our findings may assist in individualized risk assessment for SGA and LGA births.