The prognosis for 45,X/46,XX mosaicism diagnosed prenatally has yet to be established. We report our experience with 12 patients in whom prenatal diagnosis of 45,X/46,XX mosaicism was detected by amniocentesis for advanced maternal age or decreased maternal serum alpha-feto protein and compared them with 41 45,X/46,XX patients diagnosed postnatally. The girls in the prenatal group range in age from 3 mo to 10 years. All have had normal linear growth. Four had structural anomalies including: ASD (n = 1); ptosis and esotropia (n = 1); labial fusion (n = 1); and urogenital sinus, dysplastic kidneys, and hydrometrocolpos (n = 1). Gonadotropins were measured in seven; one had elevated luteinizing hormone/FSH at 3 mo of age. One has developmental delay and seizures as well as ophthalmologic abnormalities. None would have warranted karyotyping for clinical suspicion of Turner syndrome. The prevalence of 45,X/46,XX mosaicism is 10-fold higher among amniocenteses than in series of postnatally diagnosed individuals with Turner syndrome, which suggests that most individuals with this karyotype escape detection and that an ascertainment bias exists toward those with clinically evident abnormalities. The phenomenon of a milder phenotype for the prenatal group is similar to that observed for 45,X/46,XY diagnosed prenatally. Prenatal counseling for 45,X/46,XX in the absence of such ultrasound abnormalities as hydrops fetalis should take into account the expectation of a milder phenotype (except, possibly, with respect to developmental delay) than that of patients ascertained postnatally. The same does not hold true for 45,x diagnosed prenatally.
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