Pregnancy is often perceived as a harmonious process between mother and children. However, evolutionary speaking, the interests of a pregnant female are not always the same as the ones of her offspring; some degree of conflict even exist between them concerning, for instance, the allocation of nutritional resources. Analysing pregnancy as a not fully cooperative process enables understanding of certain phenomena that would be incomprehensible otherwise. Examples of these phenomena are the enormous amounts of foetal hormones released in maternal blood and certain complications linked to pregnancy, such as diabetes and hypertension. This paper tries to use the notions of inclusive fitness and genomic imprinting to understand the particular phenomena that take place during mammalian pregnancy. The different 'weapons' used by mothers and foetuses (placenta, hormones, ...) are described here, and ways in which their unbalance can lead to severe complications are explained.