At least 50%, and possibly as many as 77% of first trimester abortions are chromosomally abnormal. Autosomal trisomies account for most of those abnormalities, while monosomy X is the single most common chromosomal abnormality found in spontaneous abortions (20-25%). No apparent cause is found in most of the remainder. In couples with RPL the causes may be categorized as genetic, anatomic, endocrine, immune, age-related, environmental, and unexplained.
Although trisomies account for the majority of first trimester losses, the risk of recurrent aneuploidy in subsequent pregnancies is 1% or less. Therefore, recurrent losses due to genetic abnormalities are usually related to abnormalities of chromosome structure rather than abnormalities of chromosome number. The most common structural abnormality found in RPL is a translocation. Individuals with a balanced translocation, a condition in which material is exchanged between two or more chromosomes, may be phenotypically normal, but the fetus may have genetic deficiencies or duplications after meiotic segregation. This may result in a genetic complement incompatible with successful fetal development. Translocations may be robertsonian or reciprocal. In robertsonian translocations, acrocentric chromosomes (13, 14, 15, 21, 22, and 24) are fused at their centromeres. In reciprocal translocations, the centromeres are not involved. About 60% of translocations are reciprocal and 40% are robertsonian. Women are twice as likely as men to have a balanced translocation. Inversions are rearrangements within the chromosome in which a sequence of genes is reversed. Inversions are much less likely than translocations to be found in couples experiencing RPL.
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