Spontaneous Abortion Etiology

In approximately 50% of spontaneous abortions occurring during the first trimester, there is an abnormal karyotype. This incidence decreases to 20-30% in second trimester losses, and 5-10% in third trimester losses. As will be discussed below, the first trimester losses are typically autosomal trisomies or monosomy X, while later losses reflect chromosomal abnormalities seen in neonates.

Other suspected causes of spontaneous abortion account for a smaller percentage of losses. In a significant percentage of spontaneous abortions, the etiology is unknown. Infection, anatomic defects, endocrine factors, immunologic factors, and maternal systemic disease are felt to play a role in spontaneous abortion.

A. Morphologic and Genetic Abnormalities
Aneuploidy (an abnormal chromosomal number) is responsible for a large percentage of early spontaneous abortions, accounting for at least 50% of these losses.

Autosomal trisomies have been noted for every chromosome except chromosome number 1. Together, the autosomal trisomies make up just over 50% of all aneuploid losses. Trisomy 16 is the most commonly encountered trisomy in spontaneous abortions.

Monosomy X or Turner syndrome is the single most common aneuploidy in spontaneous losses, comprising approximately 20% of these conceptuses.

Polyploidy, usually in the form of triploidy, is found in approximately 20% of all miscarriages. Polyploid conceptions typically result in empty sacs or blighted ovums but occasionally can lead to partial hydatidiform moles.

The remaining half of early abortuses appear to have normal chromosomal complements. Of these, 20% have other genetic abnormalities which may account for the loss. Mendelian or polygenic factors resulting in anatomic defects may play a role. These tend to be more common in later fetal losses.