In June 2012, the European Society of Human Reproduction and Embryology issued a press release announcing that over 5 million children worldwide had been born with the help of in vitro fertilization. Although the sheer quantity is impressive in itself, an even more significant consequence of assisted reproductive technologies is that they have transformed a previously indisputable fact concerning biological kinship and motherhood. This fact is captured in the ancient Roman legal maximmater semper certa est(the mother is always certain), a principle that has been a central pillar for Euro-American kinship. Today, it is no longer certain that the birth mother will be defined as a child’s biological mother, and this gives rise to a flexibility in determining what biological motherhood really is. This becomes particularly significant when we look at third-party reproduction (involving a donor or surrogate) in which the meaning of biological motherhood is contextual: sometimes biological motherhood is determined with reference to nutrition via the blood, sometimes with reference to DNA, at other times with reference to epigenetic influences taking place within the womb. To deepen our understanding about the role that biology plays for the kinning process in the context of third-party reproduction, this essay introduces a theoretical framework of kinship grammars. It discusses how the kinship grammars of blood, genetics, and epigenetics offer different “rules” for determining the changing meaning of biological motherhood and suggests that a strategy of Wittgensteinian rule following in relation to the kinship grammar of epigenetics opens up the possibility for novel, perhaps even subversive, ways of thinking through kinship and biology. © 2016 by The University of Chicago. All rights reserved.