Every so often, among the bags of breast milk sent for donation to milk banks, clear bags of milk are found that are hand decorated and accompanied by short texts written by donating mothers. In the bank labs, the milk is poured into pasteurisation containers, and the bags are thrown away. The milk comes to the neonatal ward packed in bar-coded bottles. Both donor and the recipient are anonymous to one another. To whom are the donating mothers writing their messages? What can be learnt from their writings and drawings about their lived experiences of transitioning into motherhood? In the current study I integrate theoretical content about the transition to motherhood and theories about epistolary literature, likening the milk bags to postcards and letters. In contrast to a private letter written with ink on folded paper in a closed envelope, writing on ‘milk postcards’ is exposed and privacy is absent. ‘Milk postcards’ have a double transparency: the self is reflected in the messages and the contents of the bag—breast milk, a bodily fluid from the body of the donor. From a visual analysis of 81 photos of human-milk bags with text and drawings photographed by milk banks laboratory technicians, it appears that the milk postcards serve as a ‘third voice’ that echoes the difficulties and the joys in the transition to motherhood, and that donors experience an imagined solidarity with unknown mothers. The milk itself serves sometimes as an image and sometimes as the background for the writing, while its colour, texture and the form in which it is frozen constitute part of the text and serve as self-testimony for the mother of her capability and of her being a nurturing mother, for both her own baby and other unknown babies.