Dataset used for the unpublished paper "Pregnancy Serum DLK1 Concentrations associated with Fetal Genotype and Indices of Maternal Insulin Resistance and Secretion" and containing data from the Cambridge Baby Growth Study. Genotype data is not included for ethical and consensual reasons (apply to the investigators to request access if required). Uncompressed Microsoft 2013 (.xlsx) file (614 rows including header; 22 columns including one for a dummy identifier) containing data relevant to the publication collected as part of the Cambridge Baby Growth Study (data collection 2001-2019). All the study participants were recruited from pregnancy clinics at the Rosie Maternity Hospital, Cambridge (2001-2009) around week 15 of pregnancy. A research 75 g oral glucose tolerance test (after an overnight fast) was performed on participants around week 28 of pregnancy. DLK1 concentration measurements were made in fasting serum samples from this. Gestational diabetes was characterised according to International Association of Diabetes in Pregnancy Study Groups criteria. Insulin and c-peptide concentrations were measured using enzyme-linked immunosorbent assays and glucose by a standard laboratory technique. DLK1 was measured using an in-house Meso Scale Discovery assay. HOMA modelling was performed using the online HOMA calculator (https://www.dtu.ox.ac.uk/homacalculator/). Missing data are presented by empty cells. For further information about the study please contact Dr. Clive Petry (address below; email: cjp1002@cam.ac.uk). For further general information about the Cambridge Baby Growth Study please contact Dr. Clive Petry (Department of Paediatrics, Box 116, Addenbrooke's Hospital, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0QQ, U.K.; email: cjp1002@cam.ac.uk).