The study population was restricted to Danish-speaking women who answered the first questionnaire and who carried their pregnancy to a livebirth or a delivery after 28 weeks of gestation (n = 26,326). Twins were excluded from the study (n = 882). The study population was further restricted to those with valid information about the mother’s smoking habits during pregnancy (n = 25,102). The second questionnaire was answered by 18,598 (74 percent) women and the third by 16,500 (66 percent).