Prior to the let-down reflex, the mammary glands secrete a yellowish fluid called that has less lactose than milk and almost no fat. colostrum bilirubin prolactin meconium

Prior to let-down reflex, the mammary glands secrete a yellowish fluid called colostrum that has less lactose than milk and almost no fat. Colostrum is normally the first form of milk produced by the mammary glands immediately after the delivery of a new born. It contains large numbers of antibodies called secretory immunoglobulin that help protect the mucous membranes in the throat, lungs, and intestines of the infant. 

That is true! many fish species traveled to the u.s. because of it.