12/09/2025
Menstrual blood, the fluid shed from the uterus during menstruation, is far more complex than commonly imagined. Unlike the bright red circulating blood drawn from a vein, menstrual blood is a rich mixture of blood, endometrial tissue, cervical mucus, vaginal secretions, and immune cells.
Recent proteomic studies using advanced mass spectrometry have revealed that this fluid contains over 1,000 distinct proteins, a number significantly higher than previously thought. Of these, approximately 385 proteins appear to be unique to menstrual blood and are not detected—or are present only in trace amounts—in either peripheral circulating blood or ordinary vaginal fluid collected outside of menstruation.
These distinctive proteins largely originate from the shedding endometrium, the glandular tissue that thickens and breaks down each cycle. Many are involved in tissue remodeling, immune regulation, wound healing, and inflammation control, processes essential for the uterus to repair itself after shedding its lining.
Some notable examples include matrix metalloproteinases that help break down tissue, antimicrobial peptides that protect against infection during this vulnerable period, and specific cytokines that modulate local immune responses. Others are fragments of cellular proteins released during programmed cell death (apoptosis) of endometrial cells.
This unique protein signature explains why forensic scientists can now reliably distinguish menstrual blood from peripheral blood or traumatic injury blood at crime scenes using targeted antibody or mass-spectrometry tests. In essence, menstrual blood is not merely “blood that comes from the uterus”; it is a specialized biological fluid with its own molecular identity, reflecting the dynamic regenerative biology of the endometrial lining.
