1. Cunningham FC, MacDonald PC, Gant NF, Leveno J, Gilstrap K, editors. Multiple pregnancies. In: Williams Obstetrics. 19th ed. Norwalk: Appleton and Lange; 1993. p. 891–918.
2. Chitkara U, Berkowitz RL . Multiple gestations. In: Gabbe SG, Niebyl JR, Simpson JL, editors. Obstetrics: Normal and Problem Pregnancies. 3rd ed. New York: Churchill Livingstone; 1996. p. 821–862.
3. Benirschke K . Multiple gestation: incidence, etiology and inheritance. In: Creasy RK, Resnik R, editors. Maternal Fetal Medicine: Principles and Practice. 4th ed. Philadelphia: Saunders; 1999. p. 585–615.
4. Botting BJ, Davies IM, Macfarlane AJ . Recent trends in the incidence of multiple births and associated mortality. Arch Dis Child 1987;62:941–950.
5. Anonymous. From the Center for Disease Control and Prevention Contribution of assisted reproductive technology and ovulation inducing drugs to triplet and higher-order multiple births — United States, 1980–1997. JAMA 2000;284:299–300.