This may be particularly true in cases of double trisomy X and 18, in which fetal features are dominated by those related to trisomy 18  and those of trisomy X may be overlooked.
Typically, trisomy X is a maternal meiosis I error (63% of the time) and trisomy 18 is a maternal meiosis II error 59% of the time .
As isolated trisomyXshows no distinct malformations at birth except for minor abnormalities such as epicanthal fold and clinodactyly, trisomy X may be totally asymptomatic at birth and likely diagnosed later in life .
Wilson, "A review of trisomy X (47,XXX)," Orphanet Journal of Rare Diseases, vol.
Caption: Figure 3: Karyotype of placental tissue reveals coexistent trisomy 18 and trisomy X (a).
Trisomy X: focal chorangiosis, with no features of aneuploidy [(b) magnification x100, H&E].
Although Jeanne thought that her baby-to-be was being tested for Down syndrome or other significant disabilities, her obstetrician phoned her to let her know that the fetus had a high probability of having Trisomy X, or an extra X chromosome in a female.
Klinefelter Syndrome and Trisomy X are known as sex chromosome aneuploidy, or variations in the typical number of chromosomes.
One female in each 1000 live births has Trisomy X. A male can also have an extra Y chromosome, known as XYY syndrome.
Jeanne and her husband were referred to a genetic counselor who explained that the prenatal blood test results only predicted with a fair amount of certainty that their daughter would have Trisomy X. If they wanted to know definitively before birth, Jeanne would have to undergo amniocentesis, an invasive test using a needle through the uterine wall to draw out a sample of amniotic fluid.
After Jeanne's baby, Arianna, was born, a blood test confirmed the diagnosis of Trisomy X. Arianna's development was normal until she reached 18 months, when the pediatrician recommended an early intervention evaluation for speech and motor delays.