Approximately 1/1000 apparently normal females has a 47,XXX (trisomy X) karyotype (1).
Advanced maternal age increases risk of trisomy X, and the extra X chromosome is usually maternally derived.
Physical anomalies are rare. Menstrual irregularity and infertility sometimes occur. Affected girls may have mildly impaired intellect and may have more school problems than siblings.
Although rare, 48,XXXX and 49,XXXXX females exist. There is no consistent phenotype. The risk of intellectual disability and congenital anomalies increases markedly when there are > 3 X chromosomes. The genetic imbalance in early embryonic life may cause anomalous development.
(See also Overview of Chromosomal Abnormalities and see Overview of Sex Chromosome Abnormalities.)
Reference
1. Skuse D, Printzlau F, Wolstencroft J: Sex chromosome aneuploidies. Handb Clin Neurol 147:355-376, 2018. doi: 10.1016/B978-0-444-63233-3.00024-5
