![]() 1 This is carried out between 11+0 and 13+6 weeks of gestation, and combines ultrasound measurements, including nuchal translucency, maternal serum analytes (human chorionic gonadotropin, oestradiol, pregnancy-associated plasma protein A ) and maternal age to produce a risk score. In Australia, the most common screening modality for fetal chromosomal abnormalities is the combined first-trimester screen (cFTS). The likelihood of each of these three aneuploidies increases with maternal age. Most cases of trisomy 21, 18 and 13 arise de novo (as a spontaneous event), although in rare cases there may be a predisposing parental chromosomal rearrangement, such as a translocation. Other fetal aneuploidies are generally associated with spontaneous pregnancy loss, but some, particularly trisomy 18 and 13, can result in live births. The most common chromosomal abnormality is trisomy 21 (ie presence of an additional copy of chromosome 21), which causes Down syndrome. These can range in size from small segments of chromosomes (termed ‘microduplications’ or ‘microdeletions’) to entire chromosomes (ie aneuploidy). 1 Clinically significant fetal chromosomal abnormalities generally involve gains or losses of genetic material. It also enables them to make informed decisions about whether to proceed to diagnostic testing. Prenatal screening for fetal chromosomal abnormalities is carried out to identify women who are at higher risk of having an affected fetus. Articles in this series aim to provide information about emerging laboratory tests that general practitioners may encounter. This article is the first in a series on pathology testing.
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