Monosomy X, commonly known as Turner Syndrome, is a chromosome disorder in which a girl or woman has only one complete X chromosome. (Because a Y chromosome is needed for a person to be male, all babies with Turner Syndrome are girls.) Though girls born with Turner Syndrome usually have good odds for a normal life, the majority of babies with the condition are lost to miscarriage or stillbirth.
About 1 in every 1,500 to 2,500 newborn babies has Turner Syndrome. Yet according to research, monosomy X is present in about 3% of all conceptions, but about 99% of affected babies are miscarried or stillborn. The condition is thought to be a factor in roughly 15% of all miscarriages.
The cause of Turner Syndrome is an error in cell division that leaves the body's cells with only one fully functioning X chromosome. Usually the abnormality is already present at fertilization, originating in the sperm or the egg. In the condition called Mosaic Turner Syndrome, meaning some of the body's cells have monosomy X while others have normal chromosomes, the cause is an error in cell division during very early embryonic development. No one knows exactly what causes these cell division errors.
Turner Syndrome may be revealed as the cause of a miscarriage or stillbirth when parents pursue chromosomal testing after the pregnancy loss. In a current pregnancy, an ultrasound may reveal markers for the condition, but diagnosis can only be confirmed with genetic testing such as amniocentesis or CVS. And there have been some reports of false positives for the condition, even with amniocentesis. In newborn babies, the diagnosis can be confirmed with a blood test.
If Your Baby Has Been Diagnosed During Pregnancy:
Even though the high risk of miscarriage probably sounds scary, researchers believe that the majority of miscarriages related to Turner Syndrome occur in the first trimester. By the time the baby has reached the point of being eligible for an amniocentesis, the odds of pregnancy loss are not nearly as staggering. One study found that 91% of babies diagnosed via amniocentesis survived to birth. It can be unnerving to learn that your baby has a chromosome disorder though, so it's a good idea to get in touch with support groups or a genetic counselor to prepare.
Prognosis for Liveborn Babies:
Despite the high risk of miscarriage and stillbirth, the overall prognosis for a baby with Turner Syndrome is far from dismal after birth. There are some common health problems and physical characteristics, but girls with Turner Syndrome usually have normal intelligence without life-threatening disabilities and can lead happy, healthy lives. Many do not even find out that they have the disorder until adulthood.
Why Some Cases End in Pregnancy Loss While Others Do Not:
If you have lost a baby with Turner Syndrome, it is no doubt confusing to hear all the stories of women living with Turner Syndrome on one hand, and then on the other hand have your doctor tell you that Turner Syndrome caused your miscarriage or stillbirth. The truth is that doctors aren't completely sure why so many babies with Turner Syndrome are miscarried while others make it through pregnancy without major complications.
The most likely explanation is that there is a genetic factor in play. It could be that the majority of babies who are conceived with Turner Syndrome are missing genes necessary for life, whereas those that survive have a more complete set of genes, despite having only one X chromosome. Some researchers have theorized that babies that survive pregnancy with Turner Syndrome may have some level of mosaicism, at least during early pregnancy, that enables them to continue growing and developing.
Regardless of the explanation, if you have lost a baby with Turner Syndrome to miscarriage or stillbirth, it is normal and OK to grieve. The odds are low of the condition recurring in a future pregnancy, but a genetic counselor should be able to give you more information about any concerns you may have if you are planning to try again.
Gravholt, CH et al. "Prenatal and postnatal prevalence of Turner's syndrome: a registry study." BMJ 1996;312:16-21.
Held, K.R. et al. "Mosaicism in 45,X Turner syndrome: does survival in early pregnancy depend on the presence of two sex chromosomes?" Human Genetics 1992. 88(3):288-294.
Saenger, Paul. "Turner's Syndrome." NEJM 1996. 335:1749-1754.
Uematsu, A. et al. "Parental origin of normal X chromosomes in Turner syndrome patients with various karyotypes." American Journal of Medical Genetics May 2002 111(2):134-139.