Altercation of generations: genetic conflicts of pregnancy.

Pregnancy is traditionally viewed as a harmonious collaboration between mother and fetus. From this perspective, viviparity poses a series of problems that maternal and fetal genes work together to solve and the many complications of pregnancy are interpreted as evidence of the malfunctioning of an evolved system or of the failure of natural selection to achieve an adaptive goal. This view fails to recognize aspects of genetic conflict that lie at the heart of gestation. At least three interrelated sources of conflict can be identified: (i) conflict between genes expressed in the mother and genes expressed in the fetus/placenta (parent-offspring conflict); (ii) conflict between maternally-derived and paternally-derived genes within the fetal genome (genomic imprinting); and (iii) conflict between maternal genes that recognize themselves in offspring and the rest of the maternal genome (gestational drive).