see also:

• pregnancy
• autoimmune disease

• there is a bi-directional transfer of cells between mother and fetus in animals with a uterus

• fetuses transfer cells (“FMc”) to their mother from 4th week of gestation until delivery and while most of the cells in the blood stream are eliminated by maternal immune cells in the puerperium, many persist for decades in various tissues of the body
• their presence may be a factor in improving maternal health (participating in tissue repair and regeneration, cell replacement, and maternal homeostatic maintenance) but also, unfortunately, later, in causing many immune related conditions as well as cancers such as colorectal cancer (bowel cancer), but may play protective roles resulting in improved prognosis in breast cancer, thyroid cancer, melanoma, pancreatic cancer, bladder cancer, cervical cancer, lung cancer, lymphoma
• during typical pregnancies there are only 1 to 2 fetal cells per mL of maternal blood, with trophoblasts, monocytes, B and T lymphocytes, nucleated erythrocytes, and haematopoietic progenitors present
• this allows the current maternal blood tests after 10-11 weeks of pregnancy to detect fetal genetic abnormalities without need for invasive amniocentesis or chorionic villous sampling
• during pregnancy, the transferred fetal cells may pose a conflict of interest between maternal health and fetal health since these cells could modify postnatal maternal physiology, inducing changes in lactation, thermoregulation, maternal affection, and neural plasticity, increasing only fetal fitness.
• FMc might have a reparative action on the brain participating in angiogenesis and neurogenesis and are found in higher frequency in the brains of those with glioblastomas
• FMc could differentiate into active T lymphocytes, developing an autoimmune response against the maternal tissues (graft-versus-host reaction)
• fetomaternal cell trafficking is increased in a range of common pregnancy problems, including hyperemesis gravidarum, preeclampsia, antepartum hemorrhage, and miscarriage¹⁾
  • fetomaternal haemorrhage after first trimester termination of pregnancy results in an 80-fold increase in fetal cells in maternal blood
  • caesarean delivery has been conclusively linked with higher detection and greater concentrations of microchimeric cells in maternal blood than vaginal delivery ²⁾
• different pregnancy and birth complications, including preeclampsia, antepartum hemorrhage, and cesarean sections, might alter the functional fate of FMc
• preeclampsia is much more common during first pregnancies (4.1% vs 1.7% in subsequent pregnancies) but these protective benefits in subsequent pregnancies are partner-specific since a change in paternity overrides the reduced risk of preeclampsia conferred by a prior pregnancy
• research to date is very limited, partly as detection of FMc currently relies upon detecting persistent Y chromosome DNA in maternal tissues and thus this approach is not helpful for women with female fetuses, while the finding of FMc in maternal tissues remains controversial as to the role of chimeric cells as mainly in the development of disease or in its repair phase
• see iScience. 2021. Feto-maternal microchimerism: Memories from pregnancy

• transfer of cells between the pregnant woman and her fetus also occurs, resulting in persistence of maternal cells in offspring for many years after birth
• maternal cells are found in fetal tissues beginning in the second trimester, prior to or during key milestones in human immunological development, such as development of the thymus and acquisition of effector function in peripheral T cells
  • this may have important implications for preventing fetal immune cells from rejecting noninherited maternal alloantigens
  • human fetuses develop regulatory T cells (Tregs) that suppress a fetal T cell response to non-inherited maternal antigens (NIMA)
  • maternal microchimerism confers important immunological consequences in female offspring on the outcomes of next-generation, future pregnancies (eg. Rh +ve women)
  • preterm premature rupture of membranes appears to result in higher levels of maternal cell microchimerism, along with increased maturation of fetal dendritic and T cells and cord blood T cells from these neonates are sensitized against maternal antigens and produce tumor necrosis factor-α and interferon-γ, mediators that can contribute to the cascade of uterine contractions in preterm labor ³⁾
  • long-term survival of renal allografts is markedly improved between NIMA matched sibling donor–recipient pairings