However, because chromosomal anomalies usually lead to anatomical malformations, some may be detected during the 18- to 22-week fetal anatomical ultrasound that is part of standard obstetric care. Trisomy 13, sex chromosome aneuploidies, and microdeletion syndromes cannot be detected using the serum biomarkers that are part of traditional prenatal screening. 3, 4 The sensitivity and false-positive rates for trisomy 21 or 18 may differ slightly depending on the risk cutoff used (the threshold used to determine whether screening tests results are positive or negative ). 2 The sensitivity of MSS for trisomy 21 in the second trimester is 75% to 85% (specificity, 90% to 95%). The sensitivity of eFTS for trisomy 21 is about 90% (specificity, about 95%). To generate an overall risk result for trisomy 21 or 18, eFTS and MSS may use a combination of maternal age, ultrasound measurements, and serum biomarkers. Source: Adapted from Prenatal Screening Ontario and Trillium Health Partners. Similar to sex chromosome aneuploidies, microdeletion syndromes result in phenotypic variability depending on the location and length of the microdeletion. Microdeletion syndromes are clinically recognized, and have distinct physical, behavioural, and mental characteristics. Microdeletions can be inherited, occur randomly during formation of the egg or sperm cells, or occur early in fetal development. The risk of microdeletions is not related to maternal age. Most phenotypic effects are due to the absence of a few critical genes, or in some cases, a single gene. The exact size and location of a clinically relevant microdeletion may vary, but usually a specific critical region is involved. Chromosomal deletions that involve more than 5 million base pairs (bases are the chemical building blocks of DNA base pairs are two complementary bases that help connect two complementary strands of DNA) are usually visible under a microscope, but in the case of microdeletions, the changes in genetic material are too small (usually 1 to 3 million base pairs) to be seen without higher-resolution cytogenetic methods. Microdeletion syndromes occur when a small piece of a chromosome is deleted. Some people with sex chromosome aneuploidies show few to no symptoms or signs and may never be diagnosed, leading to underdiagnosis and lower prevalence estimates for these conditions. Sex chromosome aneuploidies are not related to maternal age, except for X chromosome nondisjunction errors, which increase with maternal age. Sex chromosome aneuploidies result from an incorrect number of X and/or Y chromosomes and result in phenotypic variability (i.e., differences in observable characteristics). People typically have two sex chromosomes in each cell: females have XX and males have XY. Affected fetuses may also have a higher risk of dying before they are born. In most cases, they also lead to a shorter life expectancy and increased risk of death. ![]() ![]() These aneuploidies lead to genetic conditions that are associated with different levels of intellectual disability, developmental delay, dysmorphic features (i.e., differences in body structure), and impairments in body systems. The risk of autosomal aneuploidies increases with maternal age. Monosomies occur when a chromosome is partially or completely lost they can also be mosaic. However, many discussed the need for improved pre- and post-test counselling and raised concerns about the quality of the information they received from health care providers about the conditions NIPT can screen for. People who had undergone NIPT were largely supportive of the test and the benefits of earlier, more accurate results. Compared with second-tier NIPT, first-tier NIPT detected more affected cases, but also led to more diagnostic tests and additional budget of $35 million per year for average-risk pregnant people in Ontario. Positive NIPT results should be confirmed by diagnostic testing.Ĭompared with traditional prenatal screening, second-tier NIPT detected more affected fetuses, substantially reduced the number of diagnostic tests performed, and slightly reduced the total cost of prenatal screening. ![]() We found limited evidence on NIPT for sex chromosome aneuploidies or microdeletions in the average-risk or general population. Compared with traditional prenatal screening, NIPT was more accurate in detecting trisomies 21, 18, and 13, and decreased the need for diagnostic testing. The clinical specificity for any trisomy was 99.9% (95% CI 99.8%–99.9%).
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