Not just DNA but also cells. Not just from fetus to mother but also vice-versa.
Chimerism is to harbor cells or DNA from a genetically different individual. Microchimerism is when few such cells are harbored, something that occurs routinely during pregnancy as a two-way street; enough cells from the mother make their way into the fetus and vice-versa to render both microchimeras (below from 1, 2). Blood transfusion is an artificial source of microchimerism.
What’s few? Typically <1 in 100000 during the 1st trimester to around 1 in 10000 at term (3). Too rare to identify cell types.
Fetal DNA can be detected in maternal blood as early as 4 to 5 weeks of gestation while both fetal cells as well as DNA can be reliably detected by seven weeks (4, 5, 6).
Free fetal DNA found in maternal circulation is usually rapidly cleared (7, 8).
Fetal cells rather than fetal DNA in the mother are more consequential since they could hang around for a long time, even years. For example, one case report found male hepatocyte progenitor cells from a pregnancy 17 years earlier in the liver of a nontransfused woman with hepatitis C (9). Were they trying to counter the damage from the infection and help heal the liver? Who knows. Sure seems that way.
Regardless such intriguing individual cases, the clinical relevance of feto-maternal microchimerism remains unclear.
Possibility of autoimmunity has been a fertile possibility to explore given much higher rates of specific autoimmune diseases in women. Could fetus to mother microchimerism have anything to do with it? Some studies suggest an association, others not; the debate remains unresolved (1, 2).
What then is the biological role and relevance of pregnancy-associated multigenerational microchimerism? Diana Bianchi proposes a plausible use (below from 10),
“…multigenerational chimerism is probably necessary for the development of the fetal immune system as well as ensuring reproductive fitness and health of the mother.”
Bibliography
1. Kinder, Jeremy M., et al. “Immunological implications of pregnancy-induced microchimerism.” Nature Reviews Immunology 17.8 (2017): 483. Immunological implications of pregnancy-induced microchimerism
2. Nelson, J. Lee. “The otherness of self: microchimerism in health and disease.” Trends in immunology 33.8 (2012): 421-427.
3. Hamada, Hiromi, et al. “Fetal nucleated cells in maternal peripheral blood: frequency and relationship to gestational age.” Human genetics 91.5 (1993): 427-432.
4. Thomas, M. R., et al. “The time of appearance and disappearance of fetal DNA from the maternal circulation.” Prenatal diagnosis 15.7 (1995): 641-646.
5. Ariga, Hiromichi, et al. “Kinetics of fetal cellular and cell‐free DNA in the maternal circulation during and after pregnancy: implications for noninvasive prenatal diagnosis.” Transfusion 41.12 (2001): 1524-1530. http://www.academia.edu/download/46065794/Kinetics_of_fetal_cellular_and_cell-free20160530-24883-1u7eao8.pdf
6. Poon, L. C. Y., et al. “Maternal plasma cell-free fetal and maternal DNA at 11-13 weeks’ gestation: relation to fetal and maternal characteristics and pregnancy outcomes.” Fetal diagnosis and therapy 33.4 (2013): 215-223. Maternal Plasma Cell-Free Fetal and Maternal DNA at 11-13 Weeks’ Gestation: Relation to Fetal and Maternal Characteristics and Pregnancy Outcomes
7. Lo, YM Dennis, et al. “Rapid clearance of fetal DNA from maternal plasma.” The American Journal of Human Genetics 64.1 (1999): 218-224. Rapid Clearance of Fetal DNA from Maternal Plasma
8. Lo, YM Dennis, et al. “Quantitative analysis of the bidirectional fetomaternal transfer of nucleated cells and plasma DNA.” Clinical chemistry 46.9 (2000): 1301-1309. http://clinchem.aaccjnls.org/content/clinchem/46/9/1301.full.pdf
9. Johnson, Kirby L., et al. “Significant fetal cell microchimerism in a nontransfused woman with hepatitis C: Evidence of long‐term survival and expansion.” Hepatology 36.5 (2002): 1295-1297. Significant fetal cell microchimerism in a nontransfused woman with hepatitis C: Evidence of long‐term survival and expansion
10. Bianchi, Diana W. “The Inadvertent Discovery of Human Fetal Cell Microchimerism.” Clinical chemistry 64.9 (2018): 1400-1401. http://clinchem.aaccjnls.org/content/clinchem/64/9/1400.full.pdf