Editor—The finding of a mixture of both 46,XX and 46,XY cells in amniotic fluid culture has been frequently described. In the great majority of cases, the finding is followed by the birth of a normal male infant, leading to a consensus that the finding is the result of contamination with maternal cells in a normal male fetus. There are, however, several other possible explanations for a 46,XX/46,XY karyotype at amniocentesis. These include the presence of cells from an undiagnosed twin pregnancy, cross contamination in the laboratory, the presence of cells from a “vanished” male twin, and true fetal chimerism.1 Reassuringly, all previously reported cases where 46,XX/46,XY has been found at amniocentesis have resulted in the birth of a child with a normal genital phenotype.

We present a case where a 46,XX/46,XY karyotype was found on amniocentesis. This resulted in the birth of an infant with true chimerism and an abnormal genital phenotype. We describe the possible outcomes when a 46,XX/46,XY karyotype is found at amniocentesis and discuss the implications for counselling.

A couple were seen for genetic counselling after an amniocentesis, performed because of advanced maternal age, showed a karyotype 46,XX[30]/46,XY[4]. Thirty clones from four independent cultures showed 46,XX cells, and four clones from two of these cultures showed 46,XY cells. In a fifth dispersed culture, of 60 cells examined, 58 were 46,XX and two were 46,XY. Laboratory error was very unlikely because the karyotype was found in two independently handled samples. An ultrasound examination showed male genitalia with normal appearance.

The couple were counselled regarding the possible clinical outcomes. It was explained that the 46,XX cells might be of maternal origin, in which case the child would be a normal male. The possibility of true chimerism was also raised. Severe genital ambiguity was considered unlikely because …