In 2010 an academic publication called for the common law born-alive rule to be substituted by a definition of personhood that includes an unborn but viable foetus. It was the author's submission that foetal viability occurs at 24 weeks' gestation. This assertion represents a wider legal tendency to attribute foetal survivability to a particular gestational week. An ambiguous legal concept of foetal viability has developed because different gestational weeks (which are all said to represent the point of viability) are being applied in different areas of law. This is problematic because it is not clear when the legal implications of personhood should benefit the unborn. Consequently, this article turns to medical knowledge and looks at the clinical definition of foetal viability in order to determine whether foetal viability can be legitimately applied in law for purposes of extending personhood. Research indicates that determining the viability of a foetus requires an individualised approach towards each pregnancy. This construction of viability moves beyond gestational age and includes the consideration of a wide range of foetal related variables, and medical and sociocultural variables. At best, foetal viability is a clinical estimate that can only be verified upon birth. Due to the fact that personhood has such far reaching legal implications, it is important that definitive demarcations are provided as to exactly when foetal viability occurs. However, medical science cannot provide these definitive lines because there are too many uncertain elements present when determining foetal viability. Thus personhood cannot be subject to the condition of foetal viability since this approach to personhood would require a "patient-specific" confirmation of viability for each and every pregnancy and this is unrealistic if not impossible.