What Are Sex Chromosome Aneuploidies?

Non-invasive prenatal screening (NIPS) tests like Juno’s Hazel Plus include prenatal screening for not only the common trisomies–trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome)--but also include analysis of X and Y chromosome material to check for sex chromosome aneuploidies. In this blog, we'll explore what sex chromosome aneuploidies are, how they can affect a baby, and ways to detect sex chromosome aneuploidies during pregnancy.

Sex chromosome aneuploidy means a person has extra or missing sex chromosomes, which are the X and Y. While females usually have 46 total chromosomes, including two X sex chromosomes, males typically have 46 total chromosomes - including one X sex chromosome and one Y sex chromosome. So, a female with a sex chromosome aneuploidy might have one or three X chromosomes. A male with a sex chromosome aneuploidy might have two Xs and one Y, or one X and two Ys. The extra or missing sex chromosome is a chance finding. People of any age can have a child with a sex chromosome aneuploidy, regardless of family or medical history, and diet and lifestyle can’t prevent or increase this chance.

Below, we will describe a few of the most common sex chromosome conditions:

• Monosomy X / Turner syndrome: People with monosomy X usually have only one X chromosome and no Y chromosome. This condition can also be called Turner syndrome, and it’s seen in females. When a baby has monosomy X, there may be fetal complications like heart defects, fluid in the lungs and chest, and even miscarriage. While less common, some babies with monosomy X have no fetal complications. As the years go by, girls with monosomy X tend to be shorter than their siblings and usually have infertility.

• XXY / Klinefelter syndrome: People with Klinefelter syndrome have two X chromosomes and one Y chromosome. This condition is seen in males – the extra X chromosome doesn’t usually cause differences in the appearance of male genitalia at birth. Many boys with Klinefelter syndrome will not have obvious physical differences from their brothers. But, as they get older, boys with Klinefelter syndrome tend to be taller and less muscular, with wider hips, less body hair, and more breast tissue. They usually have infertility unless special fertility treatment is pursued during adolescence.

• Triple X Syndrome / Trisomy X: Triple X syndrome, also known as Trisomy X, describes a female with three X chromosomes. Most girls with Triple X syndrome will not have birth defects or other noticeable features of their condition at birth. Some babies with Triple X will have low muscle tone, and as she grows up, a girl with Triple X might be a bit taller than her sisters. Most girls with this diagnosis do not have an intellectual disability but can have learning difficulties, such as dyslexia. Women with Triple X syndrome usually have typical fertility, but puberty problems and premature ovarian failure are possible.

• XYY / Jacobs syndrome: XYY syndrome, also called Jacobs syndrome, describes a male with one X and two Y chromosomes. Males with XYY syndrome often grow to be taller than their siblings, but they typically look like other baby boys at birth, and most will not have birth defects or apparent features of their condition. A diagnosis of XYY syndrome does not usually cause intellectual disability, but it can still be associated with delayed language development, dyslexia, and behavioral concerns such as attention deficit hyperactivity disorder and impulsivity.

During pregnancy, a sex chromosome condition may be suspected based on the results of non-invasive prenatal screening (NIPS) or if certain findings are seen on ultrasound. For example, Monosomy X (Turner syndrome) may be suspected if a baby’s ultrasound shows a cystic hygroma (a large, fluid-filled sac at the back of the baby’s head and neck) or coarctation of the aorta (a heart defect where the artery carrying blood from the heart to the body is too narrow). However, the only way to definitively diagnose a sex chromosome aneuploidy during pregnancy is through prenatal diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis. Suppose a sex chromosome aneuploidy is not diagnosed during pregnancy. In that case, a child can be diagnosed with the condition after birth through a simple blood test called a karyotype which looks at the number and arrangement of all of their chromosomes, including the X and Y chromosomes.

Sex chromosome aneuploidies include a range of conditions, and many babies with a sex chromosome aneuploidy will look like other baby girls or baby boys at birth and not have recognizable differences until later in life. If you’re interested in screening for sex chromosome aneuploidies during pregnancy, check out our Hazel Plus Non-Invasive Prenatal Screening (NIPS) test or speak with your healthcare provider about prenatal genetic screening and testing.

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