Spontaneous miscarriage in the first trimester is related to Chromosomal defects in the fetus. An estimated 15 to 20 percent of known pregnancies end in miscarriage, the loss of a pregnancy before the 20th week. Upto 70 percent of first-trimester miscarriages, and 20 percent of second-trimester miscarriages, are caused by chromosomal anomalies/abnormalities in the fertilized egg. Most often, it means that the egg or sperm had wrong number of chromosomes, and consequently the fertilized egg can't develop normally.