Dataset used for the paper "Associations between Maternal Iron Supplementation in Pregnancy and Changes in Offspring Size at Birth Mirror and Attenuate those of Multiple Micronutrient Supplementation" published in Nutrients and containing data from the Cambridge Baby Growth Study. Uncompressed Microsoft 2013 (.xlsx) file (986 rows including header; 72 columns for standard association analysis) containing data relevant to the publication collected as part of the Cambridge Baby Growth Study (data collection 2001-2009). All the study participants were recruited from pregnancy clinics at the Rosie Maternity Hospital, Cambridge (2001-2009) around week 12 of pregnancy (at booking clinics). Maternal iron, folic acid and multiple micronutrient supplementation in pregnancy, anaemia in pregnancy and maternal pre-pregnancy weight and height were self-reported and collected as part of pregnancy questionnaires. Blood pressure readings, diagnoses of pre-eclampsia and offspring birth weight and gestational age at birth were collected from hospital notes. Gestational hypertension was defined as either systolic blood pressure >=140 mmHg or diastolic blood pressure >=90 mmHg in any of the blood pressure readings or was recorded from the hospital notes. All other assessments of the baby’s size and adiposity at birth (including the measurement of skinfold thicknesses) were made by trained paediatric research nurses. Gestational diabetes was defined using 75g oral glucose tolerance tests around week 28 of pregnancy and International Association of Diabetes in Pregnancy Study Group criteria. Missing data are presented as empty cells. For further information about the study please contact Dr. Clive Petry (email: cjp1002@cam.ac.uk; Department of Paediatrics, Box 116, Addenbrooke's Hospital, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0QQ, U.K.).