Genetic factors are the most common causes of spontaneous abortions. From 50% to 80% of firsttrimester abortions show chromosomal abnormalities. Additionally, there are other genetic causes distinct from chromosomal abnormalities. Single-gene abnormalities are almost unexplored in spontaneous abortions, although single-gene defects are a more common cause of birth defects in liveborns than chromosomal abnormalities. Many causes of repeated abortions that are commonly classified as 'non-genetic' are actually the result of perturbations of gene products such as factor V Leiden, and other genes associated with thromboembolism, and alloimmune pregnancy loss (which may involve shared human leukocyte antigen (HLA) molecules). In this chapter, we shall restrict the discussion to the frequency and most common genetic causes of sporadic and recurrent abortions.