M ost of us want to have children. We want them to be healthy and have a good start in life. One way to achieve this goal is to use preimplantation genetic diagnosis (PGD). PGD enables people engaged in the process of in vitro fertilisation (IVF) to acquire information about the genetic constitution of an early embryo. On the basis of this information, a decision can be made to transfer embryos without genetic defects to the uterus and terminate those with genetic defects. However, is it morally acceptable to use PGD to reduce the probability of children with severe genetic diseases being born? Is the current routine use of PGD in public healthcare services to select against severe genetic diseases like anencephaly, spina bifida, cystic fibrosis and Down’s syndrome morally acceptable? These are complex questions involving a range of difficult ethical issues—for instance, critical discussions about the morality of embryo research and embryo termination. They also involve awkward conceptual issues concerning such matters as the meaning of words such as ‘‘disability’’ and ‘‘severe’’ in ‘‘severe genetic diseases’’, which will not be discussed here. In this paper I examine an argument which aims to show that efforts to prevent the birth of severely disabled children using PGD are morally unacceptable. Essentially, this argument appeals to our concern for disabled people and the belief that PGD, through a slippery slope process, will have bad consequences for them. I conclude that the argument is problematic for a number of reasons. But before I examine the argument itself, it will be helpful to separate two types of slippery slope argument since these involve different kinds of reasoning.