Sir

The view expressed in Patricia Pranke and João Carlos Silveiro's Correspondence 'Human rights cannot cover cells that were never in the womb' (Nature 458, 147; 2009) is one of the standard components in a belief system that detaches personhood from its genetic basis. (Others include birth, viability, ability to feel pain, and various developmental milestones.)

The issue is clouded by our prejudices about what looks human and by the way fetal development seems to be a continuum. This belief system is weakened by its arbitrariness, which is being exposed as technology pushes back the boundaries. The implantation argument will become irrelevant when 'artificial wombs' make ectogenesis possible. There is no reason to expect this will not be developed in the future, whether governments try to prohibit it or not — as has been predicted for many years (Canadian Royal Commission on New Reproductive Technologies Proceed With Care; Canadian Gov. Publ., 1993).