What causes miscarriages?

Determining the prevalence of miscarriage is difficult. Many miscarriages happen very early in the pregnancy, before a woman may know she is pregnant. Treatment of women with miscarriage at home means medical statistics on miscarriage miss many cases. While one study found a total miscarriage of 12%, a study using very sensitive early pregnancy tests found that 25% of pregnancies are miscarried by the sixth week LMP. After the age of 35, the risk of miscarriage increases considerably: 1 in 5 or 6. After 40, the risk increases to 1 in 3, and after 45 it is 1 in 2.