Methods Data Collection Between 1990 and 2015 several nation-wide cross-sectional studies among mothers of infants aged six month or younger were conducted during Well-Baby clinic visits.[12] Data were collected by means of a two-step procedure. Firstly, for each survey, organizations executing the Mother and Child Health Care program in the Netherlands were asked to provide names and addresses of five Well-Baby Clinics willing to participate. In our invitation to the organizations it was asked to identify Well-Baby Clinics whose clients vary in socioeconomic status. Next, each participating Well-Baby Clinic randomly distributed anonymous questionnaires with stamped, pre-addressed return envelopes to the first 20 visiting mothers of infants under six months of age. In 2007 and 2010, survey questions were included concerning alcohol consumption and smoking habits before, during and after pregnancy. The combined samples of 2007 and 2010 allowed us to study the overall pattern and find changes over time. Mothers were asked to complete the questionnaire at home and to send it back anonymously in a pre-paid envelope. The study design was in agreement with the Helsinki Declaration. The study design was approved by the Leiden University Medical Centre Medical Ethics Committee. Return of the anonymous questionnaire was accepted as implied informed consent, therefore the Committee did not require informed consent otherwise for these cross-sectional surveys.