pregnancies) creating a second arena for conflict between the mother and the fetus. The fetus will have a lower quality cut off point for spontaneous abortion than the mother. The mother's quality cut-off point also declines as she nears the end of her reproductive life, which is significantly seen in older mothers. Older mothers have a higher incidence of offspring with genetic defects. Initially, the maintenance of pregnancy is controlled by the maternal hormone progesterone, but in later stages it is controlled by the fetal human chorionic gonadotrophin released into the maternal bloodstream. The release of fetal human chorionic gonadotrophin causes the release of maternal progesterone. There is also conflict over blood supply to the placenta, with the fetus being prepared to demand a larger blood supply than is optimal for the mother (or even for itself, since high birth weight is a risk factor). This results in hypertension and, significantly, high birth weight is positively correlated with maternal blood pressure. A tripartite (fetus–mother–father) immune conflict in humans and other placentals During pregnancy, there is a two-way traffic of immunologically active cell lines through the placenta. Fetal lymphocyte lines may survive in women even decades after giving birth. See also Intrauterine cannibalism The kinship theory of genomic imprinting Intragenomic and intrauterine conflict in humans References External links Genetic conflicts in human pregnancy Parent-offspring conflict Evolutionary biology Reproduction