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Karyotypes from independent amniocenteses reflected a rare, unstable, functionally dicentric Robertsonian translocation chromosome in most cells in male Twin B who grew more slowly than the chromosomally normal female sib (Twin A). Twin B's balanced de novo Robertsonian translocation dic(13;14)(p11.1;p11.1), present in 81% of cells, underwent recurrent centromeric fission in 6 out of 30 independent colonies that explains a balanced 46,XY,-13,+fis(13)(p11.1),-14,+fis(14)(p11.1) karyotype. Aneuploidy for chromosomes 13q or 14q was present in 5% of cells. Instability of the Robertsonian translocation was evident because nine of the 30 colonies (30%) grown from single amniocytes had metaphase cells with more than one chromosome complement. Although uniparental disomy was excluded and a targeted ultrasound was normal, the couple was advised of the uncertain but real risk of abnormalities in Twin B and the risk to Twin A of terminating Twin B. The pregnancy proceeded and at 31 weeks gestation Twin A was in the 33rd percentile for size and Twin B in the 1st percentile. At 32 weeks, chromosome analysis revealed a balanced 45,XY,dic(13;14)(p11.1;p11.1) karyotype in all of Twin B's newborn cord blood cells with no evidence of fission or aneuploidy. Selection against unbalanced mitotic products of the unstable, functionally dicentric chromosome in early fetal development is proposed to result in Twin B's highly discordant small birth size. [ABSTRACT FROM AUTHOR]