Breast milk is menstrual blood redirected to the breasts and transformed after childbirth.

This belief survived because it connected several otherwise puzzling reproductive events into one physiological system.

Ancient medical writers noticed that menstruation usually stops during pregnancy and remains absent for some time during breastfeeding. Within humoral reproductive physiology, that suggested that the same material was being redirected: retained menstrual blood could nourish the fetus during pregnancy and later travel to the breasts to become milk.

Aristotle, Soranus, Galen, and later medical authorities repeated variants of this theory. It generated additional doctrines about colostrum, maternal diet, sexual activity during lactation, wet nursing, and the supposed effects of “bad milk” on infants.

The old model could not survive modern physiology. Harvey’s circulation weakened many ideas about special one-way bodily flows. Microscopy and gland anatomy established milk as a secretion produced by mammary tissue. Chemistry identified its water, fats, proteins, sugars, minerals, and cells. Endocrinology later showed how prolactin drives milk synthesis and oxytocin mediates milk ejection.

The exact endpoint is diffuse, so the card is not used for lifespan statistics. Its importance is the remarkable durability of a single explanatory bridge linking menstruation, pregnancy, and lactation for well over a millennium.