This is because the fetal and adult T lymphocytes develop from different stem cells. The fetal hematopoietic stem cells give rise to T-cell lineages that are tolerant to every antigen they encounter. It is after birth that the T cells develop from adult hematopoietic stem cells and are able to recognize antigens and mount an immune response.
Read more at Buzzle: http://www.buzzle.com/articles/why-doesnt-a-mothers-body-reject-her-fetus.html