During the Middle Ages, every part of a saint’s body could (and would) be turned into a relic for private or public worship. These so-called first-class relics stand in opposition to secondary ones that “merely" came into contact with a holy body, such as a pair of sandals believed to have been worn by an apostle.
It was therefore more lucrative for medieval relic vendors to manufacture something directly out of a human body. In the case of the Virgin Mary, however, her ascent into heaven caused significant problems given that she did not leave a body behind. In her case, vendors had to become particularly creative and sold "her" breast milk in little flasks or vials.
For this reason, hundreds of these Mary relics were made. Fun fact: the Elector of Saxony, Friedrich III (1463-1525), who, with his approximately 19,000 pieces, held one of the largest collections of relics in Northern Europe, and proudly owned several samples of the Virgin’s breastmilk.