Extensively employed in reproductive science, the term fetal-maternal interface describes how maternal and fetal tissues interact in the womb to produce the transient placenta, purporting a theory of pregnancy where mother', fetus', and placenta' are already-separate entities. However, considerable scientific evidence supports a different theory, which is also elaborated in feminist and new materialist literatures. Informed by interviews with placenta scientists as well as secondary sources on placental immunology and the developmental origins of health and disease, I explore evidence not of interfacing during pregnancy, but of intra-action, or the mutual emergence of entities in simultaneous practices of differentiation and connection. I argue that attending to evidence that can be figured as intra-action enables us to recognize, account for, and attend to diffuse responsibilities for fetal-maternal outcomes that extend beyond mothers to the biosocial milieus of pregnancy. In reimaging the intra-action of placentas, a new understanding of what constitutes a healthy pregnancy' becomes possible.