As many as 10 to 15 percent of confirmed pregnancies are lost. The true percentage of pregnancy losses might even be higher as many take place before a woman even knows that she is pregnant. Most losses occur very early on — before eight weeks. Pregnancy that ends before 20 weeks is called miscarriage. Miscarriage usually happens because of genetic problems in the fetus. Sometimes, problems with the uterus or cervix might play a role in miscarriage. Health problems, such as polycystic ovary syndrome, might also be a factor. 
After 20 weeks, losing a pregnancy is called stillbirth. Stillbirth is much less common. Some reasons stillbirth occur include problems with placenta, genetic problems in the fetus, poor fetal growth, and infections. Almost half of the time, the reason for stillbirth is not known.