Detection of pericentric inversion of X chromosome in a male fetus

Amniocentesis on a 32-year-old woman at risk for trisomy 21 by maternal serum triple screen showed a 46,Y,inv(X) (p22.1q24) karyotype in all cells analyzed. A blood sample was obtained from the mother for cytogenetic evaluation. Since she had the same inversion, DNA replication studies were performed to determine if the X inactivation pattern was random or not, since skewed inactivation of the inverted X might suggest that the breakpoints disrupted functional genes. DNA replication studies demonstrated that 68% of mother's cells with the inverted X were active, suggesting random X inactivation. The random X inactivation pattern suggested that the inversion is probably balanced and should not affect the fetus. A normal male was delivered at 40 weeks gestation.