sentence2: "Three Decades of Twin Births in the United States, 1980–2009" , National Center for Health Statistics Data Brief, No. 80, January 2012 Zach, Terence (2007-10-02).
sentence1: "Three Decades of Twin Births in the United States, 1980–2009" , National Center for Health Statistics Data Brief, No. 80, January 2012 نسخة محفوظة 18 ديسمبر 2017 على موقع واي باك مشين.