13 Risk of other anomalies, including single gene defects and central nervous system, cardiac, skeletal, and abdominal wall defects, is also significantly increased in these pregnancies. 9 A subsequent observational study confirmed increased prevalence of cardiac defects in patients with a nuchal translucency greater than 3.5 mm with chromosomally normal pregnancies. 4, 9– 12 In the initial observational study describing this phenomenon, 35% of patients with a nuchal translucency measurement greater than 3 mm subsequently had confirmed aneuploidy. 4Ī nuchal translucency of greater than 3 mm is significantly associated with both aneuploidy and structural malformations. The detection rate for trisomy 21 varies from 82% to 87% depending on the laboratory, using a 5% screen positive rate. One in 300 is commonly used as the cutoff for a high-risk result, but the cutoff is laboratory dependent. This risk estimate is then expressed as a ratio, such as 1 in 10. Some risk estimators also incorporate presence or absence of visualized nasal bone. A risk estimate is then developed that incorporates maternal age, past pregnancy history, number of fetuses in the current gestation, weight, race, serum markers, and nuchal translucency measurement.
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Serum markers, including free beta–human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A, are collected with a capillary blood sample between 9 and 13 weeks 6 days’ gestation. The first-trimester screen is a commonly used screening test that includes a combination of serum screening and ultrasonographic examination of the nuchal translucency performed between 10 and 13 weeks 6 days’ gestation. 8 This idea was revolutionary at the time, as previously only women who were considered to be at high risk had been offered these tests. 77,” which recommended making aneuploidy screening or invasive testing available for all women, ideally at their first prenatal visit. In 2007, the American Congress of Obstetricians and Gynecologists (ACOG) released “ACOG Practice Bulletin No. Subsequently, noninvasive tests, including serum analyte screening and cell-free DNA screening, were developed for purposes of screening for genetic abnormalities within a pregnancy. Chorionic villus sampling (CVS) is another diagnostic test and can be performed earlier in gestation. 7 Amniocentesis has become increasingly safe and is now used for several purposes, including genetic screening and infectious evaluations. 6 Amniocentesis, the first available prenatal chromosomal diagnostic testing option, was first described in the 1950s. Aims of ultrasonography include determination of gestational age and fetal number, evaluation for malformations, testing of fetal well-being, and assistance with invasive diagnostic and therapeutic procedures. Real-time gray-scale imaging became available in the 1970s and improved prenatal diagnosis by allowing for evaluation of pregnancies earlier in gestation. Initial screening for birth defects was developed in the 1950s with ultrasound and has become increasingly prominent in obstetric care.
It is important to note that aneuploidy screening and testing decisions are heavily value driven a frank discussion of the benefits, risks, and limitations of tests is key in ensuring that care is appropriate for each patient’s individual goals. 5 All patients choosing to undergo screening or testing should receive counseling regarding risks, benefits, and limitations of their chosen testing plan from their health care provider or genetic counselor.
Some birth defects, such as some neural tube defects, may be eligible for prenatal treatment with subsequently improved neonatal outcomes.
Others may choose to pursue screening or testing to allow them time to process the diagnosis and seek experienced clinicians who may be able to aid them in preparation for caring for an affected infant and to care for their child after delivery. Some may choose pregnancy termination if the defect is identified at an early enough gestational age. Patients report many different motivations for pursuing aneuploidy screening or prenatal diagnosis.