This study was co-financed by PhD scholarships where funding was covered by the Danish Agency for Science, Technology and Innovation, Copenhagen Graduate School of Health Science (CGSHS) and the Fertility Clinic at Copenhagen University Hospital, Rigshopitalet. No competing interests are declared.
Objective: We present a case of erroneous sex determination in a newborn twin girl (twin A) due to chimerism. Case Report: Amniocentesis and ultrasound examination had pointed towards male sex of both twins. At birth, twin A presented as a phenotypically normal female with 46,XY karyotype, and 46,XY gonadal dysgenesis was suspected. Twin B was a normal male. Results: In our department, further examinations of twin A included undetectable testosterone and inhibin-B and elevated FSH. Ultrasound suspected an infantile uterus, and sequencing of the SRY gene was normal. After gonadectomy, a 46,XX karyotype was demonstrated in both normal infantile ovaries and in the fibroblasts from a skin biopsy. Analysis of X-linked markers in DNA from blood lymphocytes in both twins was identical, consistent with 46,XY karyotypes. Conclusion: Twin A is a 46,XX female with a chimeric 46,XY blood cell line due to intrauterine transfusion from her twin brother.
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