1. The outermost coat or serous layer of a visceral structure that lies in the body cavities of the abdomen or thorax; it consists of a surface layer of mesothelium reinforced by irregular fibroelastic connective tissue.
2. The outermost of the extraembryonic membranes that encloses the embryo and all its other membranes; it consists of somatopleure, i.e., ectoderm reinforced by somatic mesoderm; the serosa of mammalian embryos is frequently called the trophoderm.

Vaginal discharge occurring about 5 days after delivery: Lochia serosa is the second stage of the postpartum healing process, ensues as a thin watery fluid, and has the shade of pink or brown.