Dataset used for the paper "Increased Basal Insulin Sensitivity in Late Pregnancy in Women Carrying a Male Fetus: a Cohort Study" published in Biology of Sex Differences and containing data from the Cambridge Baby Growth Study. Uncompressed Microsoft 2013 (.xlsx) file (814 rows including header; 30 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 pre-pregnancy body mass indexes were calculated from heights and weights collected from pregnancy questionnaires. Records about anaemia and smoking during pregnancy were recorded from the same questionnaire. 75 g oral glucose tolerance tests were performed in the women around week 28 of pregnancy. Glucose and insulin concentrations were measured by a standard glucose oxidase method for laboratory glucose measurements at 0 and 60 min. post-load, an Abbott Freestyle Mini for near patient capillary glucose measurements at 120 min. post-load, and a DAKO insulin ELISA. Surrogate indices of insulin secretion and sensitivity were calculated from these concentrations. Gestational diabetes was defined using International Association of Diabetes in Pregnancy Study Group criteria. The babies’ birth weights were collected from hospital notes, having been recorded by midwives. 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 within the first two weeks of life. 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.).