Probably most personnel working within assisted reproductive technology (ART) have occasionally held concerns about a patient’s capacity to parent. This raises the question of responsibilities under the principle of the “best interests of the child”. This principle has long been enshrined in advisory guidelines as well as legislation, as though the meaning and intent are self-evident. Whilst its intent is laudable, its application is problematic. Clinical practice is patient-centred, and ART personnel are highly responsive to the desire and sense of immediacy of the infertile woman (or couple) for a child. However, over a 39-year period, our clinic has faced a number of challenging patient presentations, causing serious concerns about the potential implications and/or risks to a child conceived by ART and creating the dilemma: do we treat, or not treat? Drawing on these examples, this paper discusses the nature of “the principle”, from a variety of perspectives. It considers that the best interests of the future child are served by addressing the medical and psychosocial concerns of the potential mother, rather than engage with the ill-defined and poorly understood principle of “the best interests of the child” in legislation and deny her the opportunity for motherhood based on a perception of an unfavourable environment for a future child.