Resorbed co‐twin as an explanation for discrepant chorionic villus results: Non‐mosaic 47,XX, +16 in villi (direct and culture) with normal (46,XX) amniotic fluid and neonatal blood

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Abstract
Non‐mosaic trisomy 16 was observed in chorionic villus cytotrophoblasts (direct) as well as cultured mesenchymal core cells derived from the pregnancy of a 38‐year‐old woman. Chromosome preparations from amniotic fluid and neonatal cultures (cord blood) were 46,XX. Normal fetal growth as determined by serial ultrasound examinations occurred throughout the pregnancy, which resulted in a healthy 2724 g female. Multiple biopsies taken from the umbilical cord, placental cotyledons, and fetal membranes were 46,XX. However, a placental nodule and three of six cultures initiated from membranes (amnion and chorion) showed 46,XX/47,XX, + 16 mosaicism. We propose that the trisomy 16 cells arose from residual villi derived from a trisomic cotwin that never developed. This case further demonstrates that normal fetal growth may presage normal outcome irrespective of cytogenetic findings in cytotrophoblasts (direct) and cultured mesenchymal core cells.