We conducted the study between 1 January 1991 and 31 December 2000 in the Danish county of North Jutland and included data on all women who delivered a live infant after 28 weeks' gestation. The county birth registry contains information on all births since 1 January 1991. The main data comprise maternal age, self reported smoking status at the first visit to the midwife, birth order, gestational age, length and weight of neonates at birth, civil status, and civil registry numbers for both mother and child.
We identified all cases of infantile hypertrophic pyloric stenosis from the county hospital discharge registry, which contains data on all discharges from the hospitals since 1977, the civil registry number, dates of admission and discharge, surgical procedures performed, and up to 20 diagnoses classified according to the international classification of diseases (ICD-8 until the end of 1993 and then ICD-10). The codes for infantile hypertrophic pyloric stenosis were 750.19 (ICD-8) and Q40.0 (ICD-10). The civil registry numbers were used to link the records in both registries.
We used SAS version 8.02 for logistic regression. We estimated the risk of infantile hypertrophic pyloric stenosis among infants born to smoking mothers and non-smoking mothers adjusted for maternal age, civil status, birth order, and sex of the child.
There were 57
996 births during the study, and 16
725 (28.8%) mothers smoked. We identified 78 cases of infantile hypertrophic pyloric stenosis, yielding a cumulative incidence of 1.3%. Thirty five cases (0.2%) occurred among maternal smokers compared with 43 (0.1%) among maternal non-smokers, giving a relative risk of 2.0. The adjusted odds ratio was also 2.0 (95% confidence interval 1.3 to 3.1, table). The correlation between the proportion of smokers and annual incidence of infantile hypertrophic pyloric stenosis per birth year was 0.65 (Spearman's τ
=0.65, P=0.04). The highest incidence was 0.22% in 1991 (33% were smokers), the lowest 0.05% in 1997 (28% smokers).