21.1. GENERAL INFORMATION
Miscarriage is spontaneous termination of pregnancy (spontaneous abortion) before 22 weeks gestation. Spontaneous preterm labor is spontaneous termination of pregnancy at 22 to 37 weeks gestation counting from the first day of last menstrual period. In English language literature authors propose the following definition of abortion: any termination of pregnancy ending in birth of immature nonviable fetus.
Spontaneous miscarriage is involuntary termination of pregnancy before the time when the fetus is able to survive independently.
Miscarriage is classified into early miscarriage (before 12 weeks), late miscarriage (at 12-21 weeks) and premature birth (at 22-36 full weeks) (Fig. 21.1).
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Fig. 21.1. Classification of miscarriage and premature birth: before 12 weeks - early miscarriage; 12-21 weeks - late miscarriage; 22 to 36 weeks - premature birth
The term habitual abortion in Russian Federation is applied to spontaneous miscarriage for two times in a row at a gestational age before 22 weeks. In other countries habitual abortion means termination of pregnancy for three and more times in a row; however, it is recommended to perform investigations after two miscarriages.
Preterm birth, according to WHO, is spontaneous termination of pregnancy at gestational age from full 22 weeks to full 36 weeks, birth of a fetus with a weight more than 500 g.
21.1.1. Epidemiology
Loss of a desired pregnancy occurs in almost every fifth pregnant woman. The rate of spontaneous abortion amounts to 10-20% of clinically diagnosed pregnancies. Miscarriage is associated with obstetric history; its rate is higher in women with primary miscarriage and lower in those who has given birth (Fig. 21.2).