1. Benirschke K, Kaufmann P. Pathology of the Human Placenta. Berlin: Springer; 2000.
2. Cunningham FG, Macdonald PC, Gant NF, Leveno KJ, Gilstrap LC, Hankins GDF, Clark SL. Williams Obstetrics. 20th ed. Stamford, CT: Appleton&Lange; 1997.
3. Moore KL. The Developing Human. Philadelphia, PA: W.B. Saunders; 1998.
4. Billington WD. The immunological problem of pregnancy: 50 years with the hope of progress. A tribute to Peter Medawar. J Reprod Immunol. 2003;60:1-11.
5. Medawar P. Some immunological and endocrinological problems raised by the evolution of viviparity in vertebrates. Symp Soc Exp Biol. 1953;7:320-338.