The phenotype of an individual, including their susceptibility to disease, is governed by several factors including parental genes and intrauterine environment. Thus, the risk of developing Type 2 diabetes is modulated by the inheritance of specific genetic variants that are slowly being characterised by the techniques of linkage analysis and population association studies using either a candidate gene or genome-wide scan approach. At the same time, evidence has accrued that alterations in the nutritional status of the developing foetus also increase the risk of diabetes in later life. Restricting protein intake in pregnant dams or interfering with placental function increases the risk of diabetes in offspring and light weight babies are more likely to develop Type 2 diabetes as adults than heavier ones. The oocyte plays a key role, since it contains not only the maternal haplotype but other information such as mitochondrial DNA and factors that modulate the expression of genes in the developing foetus. Although the ovaries contain a huge number of primordial follicles, generally each month only one oocyte matures to ovulation. Little is known about the processes that control this phenomenon. Certainly, primordial follicles and oocytes are not all the same, differing especially in mitochondrial DNA content. As women age, the oocytes released are more likely to contain genetic errors, explaining the increased risk of Trisomy 21 with maternal age. It is generally assumed that primordial follicle development and the selection of a single ooctye for ovulation is a random process. This paper suggests that this may not be the case but that a carefully controlled system may allow the mother to release an oocyte that is best suited to the prevailing environment. This would represent an important mechanism for species adaptation. Many human infertility treatments involve pharmacological superovulation, egg harvesting and culture prior to in vitro fertilisation and reimplantation. These will bypass any system of controlled ovulation and therefore might alter the risk of diseases such as Type 2 diabetes mellitus in later life. Although the offspring of human infertility treatments are generally born healthy, it is important to note that the oldest "test-tube" baby is still less than 30-years old, so the risk of late-onset diseases is still unknown.