Epidemiology
Determining the prevalence of spontaneous abortions is difficult. Many happen very early in the pregnancy, before a woman may know she is pregnant. Treatment of women without hospitalization means medical statistics misses many cases. Prospective studies using very sensitive early pregnancy tests have found that 25% of pregnancies abort by the sixth week LMP (since the woman's last menstrual period), however, other reports suggest higher rates. One fact sheet from the University of Ottawa states, "The incidence of spontaneous abortion is estimated to be 50% of all pregnancies, based on the assumption that many pregnancies abort spontaneously with no clinical recognition." The NIH reports, "It is estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among those women who know they are pregnant, the miscarriage rate is about 15–20%." Clinical abortions (those occurring after the sixth week LMP) occur in 8% of pregnancies.
The risk of aborting decreases sharply after the 10th week LMP, i.e., when the fetal stage begins. The loss rate between 8.5 weeks LMP and birth is about two percent; loss is “virtually complete by the end of the embryonic period."
The prevalence increases considerably with age of the parents. One study found that pregnancies from men younger than 25 years are 40% less likely to end in spontaneous abortion than pregnancies from men 25–29 years. The same study found that pregnancies from men older than 40 years are 60% more likely to end in spontaneous abortion than the 25–29-year age group. Another study found that the increased risk in pregnancies from older men is mainly seen in the first trimester. Yet another study found an increased risk in women, by the age of 45, on the order of 800% (compared to the 20–24 age group in that study), 75% of pregnancies ended in spontaneous abortion.
Read more about this topic: Miscarriage