Sex. We walk the reader through why it matters to generate variation in a species. The mixing of genes from two sexes will be familiar to most readers as the reason for the uniqueness of each of us, right from the moment of conception. But we give insight on the other, less widely understood, ways that the differences between us come about. This leads to explanation of the early ‘conversations’ between the mother and her embryo that take place, and why they matter. We give some current and long-standing examples of mankind’s attempts to control conception or to encourage it, including the eugenic sterilisation agenda, artificial insemination and pregnancy termination after fetal sex determination. We explore one of the most hotly debated areas of medicine, assisted reproductive technologies, to which many people will relate. We encourage the reader to challenge the way they think about the preconception period, and consider why the responsibility seems to fall on girls and women, with its consequences for gender equality. This leads to the next chapter.