In 2025, an Australian couple asked to have their remaining embryos moved to another clinic, only to discover that the child they had birthed 2 years earlier had not come from their own embryos, but an embryo belonging to a different couple. These situations can lead to disputes about who is recognised as ‘the parents’ in the biological or social sense, as well as who has moral or legal claims to parental rights and responsibilities. In terms of specific legal disputes over custody or guardianship, the matter will generally be resolved in the best interests of the child. However, one of the considerations relevant to this child’s best interests is the question of biological relatedness, even if only due to the social weight it is often granted. This paper will argue that the current presumption in favour of genetics as determinative of biological relatedness is rebuttable in favour of the gestational relationship. Furthermore, there are other reasons to give weight to the moral, legal, or social claims of the gestational progenitors, such as bonds with the infant that have already been developed. However, such mix-ups will happen again and, in light of genomic technologies, may be discovered in vivo or immediately after birth, in which the courts may be ill-suited to determining the best interests. As such, legislative approaches to resolving parenthood in such cases must be proactively developed.