Human In humans (and perhaps in all placentals), the most common form is fetomaternal<unk>  (also known as fetal cell<unk>  or fetal chimerism) whereby cells from a fetus pass through the placenta and establish cell lineages within the mother. Fetal cells have been documented to persist and multiply in the mother for several decades. The exact phenotype of these cells is unknown, although several different cell types have been identified, such as various immune lineages, mesenchymal stem cells, and placental-derived cells. A 2012 study at the Fred Hutchinson Cancer Research Center, Seattle, has detected cells with the Y chromosome in multiple areas of the brains of deceased women. Fetomaternal<unk>  occurs during pregnancy and shortly after giving birth for most women. However, not all women who have had children contain fetal cells. Studies suggest that fetomaternal<unk>  could be influenced by killer-cell immunoglobin-like (KIR) ligands. Lymphocytes also influence the development of persisting fetomaternal<unk>  since natural killer cells compose about 70% of lymphocytes in the first trimester of pregnancy. KIR patterns on maternal natural killer cells of the mother and KIR ligands on the fetal cells could have an effect on fetomaternal<unk> . In one study, mothers with KIR2DS1 exhibited higher levels of fetomaternal<unk>  compared to mothers who were negative for this activating KIR. The potential health consequences of these cells are unknown. One hypothesis is that these fetal cells might trigger a graft-versus-host reaction leading to autoimmune disease. This offers a potential explanation for why many autoimmune diseases are more prevalent in middle-aged women. Another hypothesis is that fetal cells home to injured or diseased maternal tissue where they act as stem cells and participate in repair. It is also possible that the fetal cells are merely innocent bystanders and have no effect on maternal health. After giving birth, about 50–75% of women carry fetal immune cell lines. Maternal immune cells are also found in the offspring yielding in maternal→fetal<unk> , though this phenomenon is about half as frequent as the former.<unk>  had also been shown to exist after blood transfusions to a severely immunocompromised population of patients who suffered trauma. Other possible sources of<unk>  include gestation, an individual's older sibling, twin sibling, or vanished twin, with the cells being received in utero. Fetal-maternal<unk>  is especially prevalent after abortion or miscarriage. Animal<unk>  occurs in most pairs of twins in cattle. In cattle (and other bovines), the placentae of fraternal twins usually fuse and the twins share blood circulation, resulting in exchange of cell lines. If the twins are a male–female pair, then XX/XY<unk>  results, and male hormones partially masculinize the heifer (female), creating a martin heifer or freemartin. Freemartins appear female, but are infertile and so cannot be used for breeding or dairy production.<unk>  provides a method of diagnosing the condition, because male genetic material can be detected in a blood sample.
Microchimerism
Transplacental carcinogenesis
Germline mutation
Neuroplasticity
Infantilism 
Vertically transmitted infection
Confined placental mosaicism
Postzygotic mutation