WHO definition of aboion is
The National Center for Health Statistics, the Centers for Disease Control and Prevention, and the World Health Organization all define aboion as pregnancy termination or loss before 20 weeks' gestation or with a fetus delivered weighing <500 g.Reference: William's Obstetrics; 25th edition; Chapter 18; Aboion
Pregnancy termination or loss before 12 weeks of gestation
Pregnancy termination or loss before 20 weeks of gestation
Pregnancy termination or loss before 22 weeks of gestation
Pregnancy termination or loss before 24 weeks of gestation
General obstetrics