While you're full of hopes and dreams for your developing little one, you also want to be sure he or she is growing normally. That's where prenatal genetic testing comes in.
You've undoubtedly heard of Down syndrome, but you may not be as familiar with its other name: trisomy Trisomy 21, along with trisomy 18 and trisomy 13, are genetic conditions that are commonly tested for during pregnancy. So, what is trisomy? What is the difference between Down syndrome, trisomy 18, and trisomy 13? And are there prenatal screening tests that can tell you whether your baby is at risk for these chromosomal abnormalities?
We'll answer each of these questions so you have all the information you need to decide whether genetic testing is right for you. Do you remember that week in biology class where you learned about genetics? If so, now's your chance to put that knowledge to use! You may remember learning about chromosomes, which are the threads of genetic material that make us who we are.
Chromosomes are responsible for everything from our eye and hair color to which diseases we'll be susceptible to as we age. Normally, we humans have 23 pairs of chromosomes in their DNA. Trisomy is a genetic disorder in which a person has three copies of a particular chromosome instead of the usual set of two. Since scientists have numbered our chromosomes 1 through 23, the name of the condition — trisomy 21, trisomy 18, or trisomy 13 — indicates the specific chromosome that carries the abnormality.
For example, in the case of Down syndrome trisomy 21 , there are three copies of chromosome number Genetically, people with Down syndrome have an extra copy of chromosome Look at the hands. Which hand is typical for trisomy 13, trisomy 18, trisomy 21, maternal triploidy and Turner syndrome? Prospective first-trimester screening for trisomy 21 in 30, pregnancies. Am J Obstet Gynecol. Benacerraf BR. The role of the second trimester genetic sonogram in screening for fetal Down syndrome.
Semin Perinatol. Ultrasound of Fetal Syndromes. Elsevier Health Sciences. One-stop clinic for assessment of risk for trisomy 21 at weeks: a prospective study of 15 pregnancies. Ultrasound Obstet Gynecol. Fetal nose bone length: a marker for Down syndrome in the second trimester. J Ultrasound Med. The genetic sonogram: a method of risk assessment for Down syndrome in the second trimester.
Choroid plexus cysts: not associated with Down syndrome. Second-trimester sonography and trisomy Second-trimester sonography and trisomy the significance of isolated choroid plexus cysts after an examination that includes the fetal hands. Ultrasound findings and multiple marker screening in trisomy Obstet Gynecol. Chaoui R.
Prenatal ultrasound diagnosis of Down syndrome. After major malformations, soft markers, nuchal translucency and skeletal signs, a new vascular sign? Aberrant right subclavian artery as a new cardiac sign in second- and third-trimester fetuses with Down syndrome. Absence of nasal bone in fetuses with trisomy 21 at weeks of gestation: an observational study. Hum Mol Genet. Coco C, Jeanty P. Karyotyping of fetuses with isolated choroid plexus cysts is not justified in an unselected population.
Contribution of ultrasonographic examination to the prenatal detection of chromosomal abnormalities in 19 centres across Europe. Ann Genet. DeVore GR. Functional analysis of mutations in TGIF associated with holoprosencephaly. Mol Genet Metab. Clin Endocrinol Oxf ; 55 — Prenatal diagnosis of craniomaxillofacial malformations: a characterization of phenotypes in trisomies 13, 18, and 21 by ultrasound and pathology.
Cleft Palate Craniofac J. Triploidy in a twin pregnancy: small placenta volume as an early sonographical marker. Prenat Diagn. Gilbert-Barness Eed. Philadelphia, PA: Mosby-Elsevier; Evaluating the incidence and likelihood ratios for chromosomal abnormalities in fetuses with common central nervous system malformations. Detection of chromosomal abnormalities, an outcome of ultrasound screening.
The Eurofetus Team. There are three types of Down syndrome. In Mosaic Down syndrome, the extra chromosome spontaneously appears as the embryo develops. Translocation Down syndrome, which accounts for approximately five per cent of cases, is inheritable. Some of the physical characteristics of Down syndrome may include:. All people with Down syndrome will experience some delay in their development and some level of learning disability.
Learn more about Down syndrome. In Victoria, Edward syndrome affects about one in 1, pregnancies. Edward syndrome is also known as Trisomy 18, because the person has three copies of chromosome 18 instead of two.
Some of the characteristics of Edward syndrome may include:. In Victoria, Patau syndrome affects around one in 3, pregnancies. Patau syndrome is also known as Trisomy 13, because the person has three copies of chromosome 13 instead of two.
Some of the characteristics of Patau syndrome may include:. Sometimes, signs of trisomy conditions may be evident during the pregnancy. Some of these signs may include:. If your child has been diagnosed with a trisomy condition, it may be helpful to speak to a genetic counsellor. Genetic counsellors are health professionals qualified in both counselling and genetics.
Genetic counsellors are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs. The Genetic Support Network of Victoria GSNV is connected with a wide range of support groups throughout Victoria and Australia and can connect you with other individuals and families affected by trisomy conditions.
Trisomies affecting the sex chromosomes—in which genetic females typically have two X chromosomes XX and genetic males have an X and Y chromosome XY —tend to be less severe. In addition to birth defects, trisomies can undermine the viability of a pregnancy. In fact, it is believed that more than half of all miscarriages are directly associated with a chromosomal defect. Of these, many are due to trisomies. No one knows for sure why chromosome 21 is so vulnerable to trisomy.
Of all the trisomies identified by researchers, Down syndrome is known to affect nearly one of every births worldwide. Edwards syndrome trisomy 18 is rare, affecting only one of every 5, births. Edwards syndrome is characterized by low birth weight, an abnormally small head, and defects in the heart, kidneys, lungs, and other organs.
While a few children with Edwards syndrome survive to adolescence, the majority die within the first year and often the first days of life. Patau syndrome trisomy 13 is the third most common autosomal disorder among newborns after Down syndrome and Edwards syndrome.
Most cases are related to a full trisomy; a very small proportion is caused by translocation or a similar condition known as mosaicism in which the chromosomal building blocks are rearranged. Children with Patau syndrome will often have cleft lips and palates, extra fingers or toes, heart defects, severe brain abnormalities, and malformed or rotated internal organs.
The severity of symptoms is such that a baby with Patau syndrome rarely lives past the first month. Warkany syndrome trisomy 8 is a common cause of miscarriage and usually results in newborn death within the first months.
Babies born with Warkany syndrome typically have a cleft palate, distinctive facial features, heart defects, joint malformation, abnormal or missing kneecaps, and an abnormally curved spine scoliosis. Full trisomy 16 is incompatible with life. While most fetuses with this abnormality are spontaneously aborted by the 12th week of gestation, a few have survived into the second trimester.
By contrast, the chances of survival of children with mosaic trisomy 16 used to be considered bleak with most deaths occurring in early infancy. Advances in genetic research have since shown that some children previously unidentified with mosaic trisomy 16 have no abnormalities of any sort and that the risk of miscarriage and birth defects is directly related to the number of cells carrying the chromosomal mutation.
With that being said, more than half of babies with mosaic trisomy 16 will have fetal abnormalities, including musculoskeletal defects, distinctive facial features, undersized lungs, and an atrial septal defect a hole between the upper chambers of the heart.
Males will often have hypospadias in which the opening of the urethra develops on the shaft of the penis rather than at the end. Development delays may occur but are less common than other trisomies. Trisomy 22 is the second most common chromosomal cause of miscarriages.
Survival beyond the first trimester is rare in babies with full trisomy The severity of physical and organ defects is such that babies carried to term are unable to survive for more a few hours or days. Some babies with mosaic trisomy 22 do survive.
The severity of birth defects is determined by the number of cells with the mutated chromosomal copy. Characteristic detects include heart abnormalities, kidney problems, intellectual disability, muscle weakness, and cognitive and developmental delays. Trisomy 9 is a rare disorder in which a full trisomy is usually fatal within the first 21 days of life. Newborns with trisomy 9 will have a smaller head, distinctive facial features including a bulbous nose and sloping forehead , a deformed heart, kidney problems, and often severe muscle and skeletal malformations.
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