Turner syndrome | Prenatal Screening Ontario

What is Turner syndrome?

Each of us is usually born with two sex chromosomes. If you are a female, you were likely born with two X chromosomes. Females with Turner syndrome have only one complete X chromosome. They are missing all or a portion of their other X chromosome.

Turner syndrome is one of the more common types of sex chromosome differences that can happen.

How does Turner syndrome happen?

Turner syndrome generally happens by chance very early in the development of the baby. Most of the time, Turner syndrome is not passed down from other family members. The chance of having a baby with Turner syndrome does not increase as you get older.

How often does Turner syndrome happen?

About 1 in 2,500 girls are born with Turner syndrome. Turner syndrome can be mild and women can live their entire lives not knowing they have it, so this condition may be more common than we think.

Types of Turner syndrome

There are many different types of Turner syndrome that can happen:

Monosomy X (45,X)

Monosomy X is the "classic" type of Turner syndrome. Each cell has only one X chromosome instead of two. About half the people with Turner syndrome have this form. It happens randomly early in pregnancy, and it is not passed down from other family members.

Mosaic Turner syndrome (45,X mosaicism)

Mosaic Turner syndrome makes up about a fifth of cases. Females with this form have a missing X chromosome in only some of the body's cells. These individuals have a mixture of cells: some of the cells have two X chromosomes, while other cells have only one typical X chromosome. The impact on a baby's development varies widely and depends on the number of cells that have the missing X chromosome material. Mosaic Turner syndrome happens randomly early in pregnancy, and it is not passed down from other family members.

Other forms of Turner syndrome

Other forms of Turner syndrome are possible. In one form of Turner syndrome, some cells have one copy of X chromosome while other cells have one copy of the X chromosome and part of the Y chromosome. In rare cases, babies have an inherited form of Turner syndrome, meaning it was passed on from the egg or the sperm.

Monosomy X (45, X)

The information below is about females with the classic form of Turner syndrome (monosomy X). Females with mosaic Turner syndrome often have milder symptoms.

What does monosomy X mean for a baby?

During pregnancy, monosomy X can be very severe. Many pregnancies with monosomy X end up in miscarriage or stillbirth (pregnancy loss later in the pregnancy). If babies with monosomy X do make it to birth, they often do well and do not generally have life-threatening issues.

Sometimes, there are clues during the pregnancy that tell us the baby could have monosomy X or another chromosome difference. Diagnostic testing can then be done so that you know for sure whether the baby has a chromosome difference or not.

| Prenatal screening tests

You might choose to have Non-Invasive Prenatal Testing (NIPT) as a prenatal screening test in your pregnancy. This test can tell you the chance for having monosomy X or another sex chromosome difference. NIPT is covered by OHIP in certain situations. Keep in mind that screening for monosomy X is not as accurate as it is for other conditions. Many people who get a "high risk" result do not end up having a baby with a sex chromosome difference.

Monosomy X cannot be screened through other OHIP-funded screening tests - enhanced First Trimester Screening (eFTS) and Second Trimester Screening (STS).

| Ultrasound

A pregnancy with monosomy X may show certain signs on ultrasound, most commonly:

More fluid at the back of the neck compared to other babies. This is often seen at the time of the 11-14 week (nuchal translucency) ultrasound. Although it is not a birth defect, having this extra fluid is a sign that the baby might have a chromosome difference.
Extra amount of fluid in other parts of the baby’s body . This is called "hydrops fetalis" and it is a very severe condition. Many pregnancies with hydrops fetalis do not survive until the end of the pregnancy.
Heart defects. Heart defects can be mild to more severe. Some heart defects are missed during the pregnancy, but can be diagnosed after birth.
Kidney defects.

| Diagnostic testing

Although the above tests can tell you there is an increased chance for the baby to have Turner syndrome, the only way to know for sure is through a diagnostic test. This testing can be done by looking at cells from the placenta (chorionic villus sampling) or from the fluid around the baby (amniocentesis).

Some people do not wish to have diagnostic testing during the pregnancy. In that case, diagnostic testing can be done after birth by getting some blood from the umbilical cord, or through a regular blood draw from the baby.

What does monosomy X mean after birth?

The symptoms of someone with monosomy X vary widely from person to person.

Newborns with monosomy X might have some physical differences at birth. This includes an unusually short neck with extra folds of skin and swelling of the hands and feet at birth. As they grow up, girls with monosomy X are often shorter than average, and this can be noticeable by age 5. At puberty, they do not have the usual growth spurt. If started early, hormone treatment helps improve the growth to near normal height.

Another symptom of monosomy X is having ovaries that do not function properly. This means that ovaries are not able to make sex hormones at puberty. Girls with monosomy X do not start their periods or develop breasts at the regular time without hormone treatment. Most females with monosomy X cannot have their own children without fertility treatment.

Females with monosomy X have an increased chance for other health issues, that can also happen in the general population:

Heart defects (present at birth). Problems with the structure of the heart happen in about half of the girls born with monosomy X. A common heart problem is coarctation of the aorta (when part of the aorta is too narrow).
High blood pressure.
Osteoporosis (weak bones).
Hearing problems, including frequent ear infections and hearing loss.
Scoliosis (when the spine curves sideways) .
Kidney defects. These happen in almost half of the cases. It does not always mean that the kidneys are not working, but there can be problems with the urine flow, leading to kidney infections.
Thyroid problems.

In general, girls with monosomy X do not have an intellectual disability. However, they may have trouble with:

Problem solving, like math.
Seeing how objects relate to each other in space.
Social interaction.
Managing and planning, memory and attention.

Regular check ups with different specialists, treatment and support greatly help with the challenges that females with Turner syndrome might face.

More information

You can get more information from other medical and Turner syndrome advocacy organizations. These can be helpful resources when looking for a range of views on living with this chromosome difference and when you are looking for information in other languages.

Turner Syndrome Society of Canada

Mayo Clinic - Turner syndrome (available in English, Simplified Chinese and Arabic)

Cleveland Clinic - Turner syndrome