A developing fetus is a foreign body to its mother, but the mother’s immune system has an immunological tolerance to it. Conversely, the human fetal immune system, which develops at about 10 weeks, recognizes and tolerates non-inherited maternal material.
Blood is produced in waves during fetal development; each wave generating distinct populations of T cells that coexist for some duration, with tutoring so that the young T cells learn from their elders. The development of the human immune system is a tiered process.
Early in development, T cells come from the liver, and are biased toward immune tolerance. Later, hematopoiesis switches to fetal bone marrow. The ratio of T cells gradually moves from a higher population of regulatory T cells to a ratio of regulator/enforcer T cells found in adults.
There is hazard in the fetal immune system bias toward tolerance during its formative education. Exposure of a fetus to malaria in utero, though the placenta, results in an expansion in regulatory T cells that are malaria specific. That is why children infected from placental malaria are more susceptible to malaria later in life.