Among cases that are detected by routine amniocentesis or chorionic villus sampling, one study found that the prevalence of Turner syndrome among tested pregnancies was 5.58 and 13.3 times higher, respectively, than among live neonates in a similar population. Cause
Turner syndrome is caused by the absence of one complete or partial copy of the X chromosome in some or all the cells. The abnormal cells may have only one X (monosomy) (45,X) or they may be affected by one of several types of partial monosomy like a deletion of the short p arm of one X chromosome (46,X,del(Xp)) or the presence of an isochromosome with two q arms (46,X,i(Xq)) Turner syndrome has distinct features due to the lack of pseudoautosomal regions, which are typically spared from X-inactivation. In mosaic individuals, cells with X monosomy (45,X) may occur along with cells that are normal (46,XX), cells that have partial monosomies, or cells that have a Y chromosome (46,XY). The presence of mosaicism is estimated to be relatively common in affected individuals (67–90%).The (46,X,i(Xq) isochromosome in the Turner syndrome is classified as an small supernumerary marker chromosome (sSMC). Two of the types of sSMCs in this syndrome contain parts of the genetic material from either an X or, much less frequently, Y chromosome and may or may not contain an XIST gene. In normal females, the XIST gene occurs on the X chromosome inherited from her mother but not on the X chromosome inherited from her father.
: 84 In girls with Turner syndrome who do not experience spontaneous puberty, exogenous estrogen is used to induce and maintain feminization. Estrogen replacement is recommended to begin at around age 11–12, although some parents prefer to delay the induction of puberty in girls with lower social and emotional preparedness. The dose of estrogen in induced puberty begins at 10% of adult estrogen levels and is steadily increased at six-month intervals, with a full adult dose attained two to three years after the beginning of treatment. Estrogen replacement may interfere with growth hormone therapy, due to the closing effects of estrogen on growth plates; individuals must weigh up their preferences for taller height versus greater feminization. : 97–103

Fertility
Women with Turner syndrome are infertile. Only 2%–5% are capable of pregnancy without fertility treatment, most with mosaic karyotypes. Early in gestation, fetuses with Turner syndrome have a normal number of gametes in their developing ovaries, but this starts decreasingly rapidly as early as 18 weeks of pregnancy; by birth, girls with the condition have markedly reduced follicular counts. Women with Turner syndrome who wish to raise families but are incapable of conception with their own oocytes have the options of adoption or of pregnancy with donor eggs; the latter has a comparable success rate to donor pregnancy in women with 46,XX karyotypes.Pregnancy in Turner syndrome is inherently high-risk; the maternal death rate is 2%.Usually, estrogen replacement therapy is used to spur the growth of secondary sexual characteristics at the time when puberty should onset.