Soon two American biotechnology firms hope to offer couples undertaking in vitro fertilisation the chance to screen embryos before they are implanted. Pre-implantation genetic diagnosis is already widely used to test for chromosomal abnormalities or specific genetic disorders. But MyOme and Genomic Prediction plan to reconstruct the whole sequence of an embryo’s genome using just a few cells from a biopsy and genetic sequences of both parents. They can then, in theory, calculate the risk the embryo will develop a wide range of different diseases in later life—including ailments that are extraordinarily complicated, involving thousands of genetic variants. By selecting between different embryos, those undergoing IVF can optimise the health of their progeny in a way that those who conceive naturally cannot. That raises ethical concerns. Although both firms will screen embryos for disease risk only, there is no reason why traits such as height or intelligence might not be selected in the same way.