Key words: ultrasound, prenatal diagnosis, triploidy, Turner syndrome. SUMMARY. . Trisomy 13 – Trisomija .. my, 19 Turner syndrome, 13 trisomy, and 10 oth- ers. date, with significantly poorer outcome compared with. with trisomy 21 the nasal bone is not visible at the 11–13+6 weeks scan and examination of the fetus is alive, accurate dating of the pregnancy, early diagnosis of major fetal .. In trisomies 18 and 13 and Turner syndrome, the rate of fetal. Babies with Trisomy 13 and 18 are associated with more severe intellectual and NIPT has revolutionised the way we screen for Down syndrome (Trisomy 21) and 13) and sex chromosome conditions such as Turner syndrome ( Monosomy X) . will be lower when the pregnancy is less advanced than expected by dates.
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Indeed, the emerging picture indicates that aneuploidy is not due to a single causal factor but involves a complex constellation of effects that begins in utero, continues throughout the reproductive lifespan of the woman, is exacerbated by age and is facilitated by the unique features of cell cycle control in the oocyte.
The errors that lead to aneuploidy almost always occur in the oocyte but, despite intensive investigation, the underlying molecular basis has remained elusive. Increased maternal age and altered number and location of recombination events have been found to be associated with maternal meiotic errors involving chromosome 21 [ 65 ].
The overwhelming majority of trisomy 21, or Down syndrome, is caused by the failure of chromosomes to separate properly during meiosis, also known as chromosome nondisjunction. As nondisjunction is the leading cause of pregnancy loss, mental retardation and birth defects, it is imperative that we understand the biology underlying this phenomenon. Characteristics of chromosome 21 nondisjunction are typical of many of the other human autosomes. That is, the overwhelming majority is due to errors during oogenesis: One set of observations provides evidence for specific recombination patterns being the proximal cause of nondisjunction, while the others suggest an interaction between specific recombination patterns and maternal age-related risk factors.
- Nuchal scan
- Double Aneuploidy in Down Syndrome
- Trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T’s. Look at the hands
Specifically, the absence of recombination or the presence of a single recombinant event near the telomere of 21q are associated with maternal meiosis I MI errors and these associations appear to be independent of the age of the oocyte i. MII errors are associated with the placement of a recombinant event near the centromere of 21q and this association increases with increasing age of the oocyte. Nondisjunction could be possibly attributed to genetic, environmental or combined factors.
Theoretically genes predisposing to increased nondisjunction can be classified in several different ways: The occurrence of double aneuploidy would not prove the existence of predisposition gene s.
Nuchal scan - Wikipedia
Familial double aneuploidy is very rare. However, the occurrence of aneuploidy for different chromosomes is better evidence for genetic predisposition although environmental factors could also be invoked as a possible cause.
Amniocentesis and live birth data provide little evidence for a strong double aneuploidy effect although a weak effect cannot be excluded. Studies in abortions are suggestive of genetic mosaicism in double aneuploidy. Autosomal double trisomies are observed in spontaneous abortions but are rarely reported in live born infants. Most double aneuploidies are associated with an increased maternal age, abnormal sonogram, and pregnancy loss at a very early gestational age.
Triploidy Other defects with normal karyotype[ edit ] In fetuses with a normal number of chromosomes, a thicker nuchal translucency is associated with other fetal defects and genetic syndromes. The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head neither hyperflexed nor extended, either of which can influence the nuchal translucency thickness.
It is important to distinguish the nuchal lucency from the underlying amniotic membrane. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.
Further, other, non-trisomic abnormalities may also demonstrate an enlarged nuchal transparency. This leaves the measurement of nuchal transparency as a potentially useful first trimester screening tool. Abnormal findings allow for early careful evaluation of chromosomes and possible structural defects on a targeted basis.
Test accuracy and reliability is reduced at low fetal fractions. Fetal fraction will be lower when the pregnancy is less advanced than expected by dates, when the pregnancy has failed to grow, in women who have a high Body Mass Index BMI or due to other maternal factors.
The MSS must be taken before 13 weeks and 6 days of pregnancy.
Trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T’s. Look at the hands
The ultrasound uses measurement of Nuchal Translucency NTa developmental layer of fluid behind the fetal neck which is present from 11 to 14 weeks, together with other ultrasound markers for chromosomal abnormalities.
It provides an individualised risk for each fetus in multiple pregnancy and also provides a risk for Trisomy 18 and Ultrasound Alone Ultrasound uses measurement of NT between weeks of pregnancy, together with other ultrasound markers for chromosomal abnormalities, to calculate a statistical risk for Down syndrome.
This is based on extensive research by the Fetal Medicine Foundation www. Which test for Down syndrome should I have? There are 3 test available for Down syndrome: CVS and Amniocentesis-These tests are considered diagnostic. Invasive, or needle testing, remains the gold standard for Down syndrome and all other chromosomal abnormalities. NIPT is not able to provide a definite result for Down syndrome. It does not assess all the chromosomes as is done with CVS or Amniocentesis.
This is called contingency screening and may be a more cost-effective approach for younger mothers. For women with a high risk CFTS where the Nuchal Translucency and fetal anatomy are normal on ultrasound, NIPT can offer increased screening accuracy and provide the option of avoiding invasive testing with a higher level of reassurance. For those women who seek definitive diagnosis — invasive testing is the answer. What are the limitations of NIPT? Low risk result on NIPT but ultrasound shows a structural abnormality- If the ultrasound shows a structural physically visible abnormality at any stage during the pregnancy, you will likely be offered an invasive test despite having had NIPT.