colostrum FMA:62101 GAID:1159 MESH:D003126 Wikipedia:Colostrum MA:0002551 The thin, yellow, serous fluid secreted by the mammary glands during pregnancy and immediately postpartum before lactation begins. It consists of immunologically active substances, white blood cells, water, protein, fat, and carbohydrates. BTO:0000276 UMLS:C0009413 CALOHA:TS-0167 NCIT:C32348 mammary gland fluid/secretion