“… the mechanisms responsible for the paradoxical survival of the conceptus as an intra-uterine allograft within the immunologically competent genetically alien female host.”
source (Journal of reproductive immunology), October 2003 Volume 60, Issue 1, Pages 1–11
Or, put more simply 'Why doesn't an expectant mother's immune system damage her growing baby - since it has 'foreign' DNA?'
This fundamental phenomenon has yet to be fully explained.
“Pregnancy represents a major challenge to immunologic tolerance. How the fetal 'semiallograft' evades maternal immune attack is unknown.”
source (Journal of the American Society of Hematology, 2011 vol. 117 no. 6 pp 1861-1868)