Maternal smoking and age. Effect on birthweight and risk for small-for-gestational age births.

Study Objective: To analyse the information on maternal smoking routinely recorded during antenatal care by general practitioners, with emphasis on the impact of smoking on birth weight and risk for small-for-gestational age (SGA) births.

Design: Geographically based cohort study.

Setting: Evaluation of information recorded by general practitioners in antenatal records, in routine clinical practice without intervention.

Material: Women residing in one Norwegian county delivering during a 12 month period 1988/1989.

Main Results: 1908 women gave birth. 30% were recorded as smokers at first antenatal visit, 4% lacked information on smoking. The recording of smoking habits in week 36 was deficient as data were missing in 75% of the women who smoked at first antenatal visit. Smokers experienced a mean birthweight impairment of 182 grams (adjusted for parity and age). A dose-response effect on birth weight of the number of cigarettes smoked per day at first visit was found. Also, a synergistic effect of increasing maternal age and smoking with regard to risk for SGA was demonstrated. The relative risk for SGA for smokers below 25 years was not significant, whereas women of 35 years and more had a relative risk of 3.8 for SGA. 18% of the SGA-cases among the study subjects were attributable to smoking.

Conclusions: Using the routinely collected information on smoking at first antenatal visit, the well known effects of smoking on birthweight are reproduced. The routine recording of smoking habits in week 36 is insufficient. Information on smoking registered in the antenatal records should be recorded in the Medical Birth Registry. Smoking in pregnancy appears not to be equally harmful for all women. The highest risk for SGA is found in the age group where smoking is most frequent. The question of a more differentiated approach to the problem of smoking cessation in pregnancy, is raised.