Determining the precise rate is not possible as a large number of miscarriages occur before pregnancies become established and before women are aware they are pregnant. In addition, women with bleeding in early pregnancy may attend for medical care more often than women not experiencing bleeding. Some studies have attempted to account for this by recruiting women who are planning pregnancies and testing for very early pregnancy, although these would also not be representative of the wider population. A systematic review found that the cumulative risk of miscarriage between 5 and 20 weeks of gestation varied from 11% to 22% in studies assessing miscarriage rates.Up to the 13th week of pregnancy, the risk of miscarriage each week was around 2%, dropping to 1% in week 14 and reducing slowly between 14 and 20 weeks.
The prevalence of miscarriage increases with the age of the mother and the father.In a Danish register-based study where the prevalence of miscarriage was 11%, the prevalence rose from 9% in women at 22 years of age to 84% by 48 years of age