Knowledge of the condition being tested for is increasingly acknowledged as an important factor in prenatal testing and screening decisions. An analysis of the way in which family members living with an inheritable condition use and value this knowledge has much to add to debates about whether and how this type of knowledge could be made available to prospective parents facing screening decisions. This article reports on in‐depth interviews (conducted between 2007 and 2009) with 61 people with a genetic condition, spinal muscular atrophy (SMA) in their family. Many participants reported that their intimate familial knowledge of SMA offered them valuable insights with which they could imagine future lives. Other participants, however, found themselves trapped between their experiential knowledge of SMA and their (often) competing responsibility to maintain the wellbeing of their family. Still, others established a hierarchy of knowledge to rank the authenticity of different family member's accounts of SMA in order to discredit or justify their decisions. This article highlights the way in which experiential knowledge of the condition being tested for cannot be unproblematically assumed to be a useful resource in the context of prenatal testing decisions and may actually constrain reproductive decisions.