First-trimester screening provides early testing for aneuploidy between 11 and 13 weeks. Screening includes biochemical markers for pregnancy-associated plasma protein A (PAPP-A), free beta human chorionic gonadotropin (β-hCG), and ultrasound measurement of fetal nuchal translucency (NT). The detection rate for first-trimester screening is 80%, with false-positive rate of 5%. First-trimester combined screening can also screen for trisomy 18, and NT alone may detect trisomy 18, trisomy 13, and other chromosomal abnormalities. The maternal triple or quadruple screen tests are offered to pregnant women during their second trimester to identify risks for chromosome disorders such as Down syndrome (trisomy 21); major birth defects, including open neural tube defects such as spina bifida; placental insufficiency; and oligohydramnios. The evaluation consists of three or four separate blood protein tests done on maternal serum between 15 and 21 weeks of gestation: MSAFP is decreased in Down syndrome and neural tube defects, unconjugated estriol (uE3) is decreased in Down syndrome, and β-hCG is increased in Down syndrome. Results are reported as multiples of the median (MoM). The triple screen detection rate for Down syndrome is approximately 69% (false-positive rate of 5%), 80% for neural tube defects, and 60% for trisomy 18 (false-positive rate of 0.2%). The detection rate for Down syndrome with quadruple screen is 81%. Combining first and second-trimester screening improves the detection rate to 94%–96%, with a 5% false-positive rate.
The maternal triple and quadruple (Quad) tests are screening tests; therefore, an abnormal (positive) result is not diagnostic, and further testing with ultrasound, amniocentesis, and genetic counseling is indicated. The markers can be positive in normal variations such as multiple births or miscalculated gestational age. The triple screening test includes alpha-fetoprotein (AFP), hCG, and uE3. The Quad test includes the triple screening tests plus inhibin A.
Ultrasound testing is a method of assessing fetal well-being that has become a diagnostic tool for assessment of fetal age, health, growth, and identification of anomalies. Level I ultrasound assesses gestational age, number of fetuses, fetal death, and the condition of the placenta. Level II ultrasound assesses specific congenital anomalies or abnormalities. In some diagnostic centers, fetal echocardiography is also available. An additional ultrasound marker for Down syndrome is the absence or hypoplasia of nasal bones in fetuses at 11–14 weeks’ gestation. When combining this marker with PAPP-A, β-hCG, and NT, the detection rate for Down syndrome is significantly increased. Color-enhanced Doppler ultrasound is used to measure the velocity and direction of blood flow in fetal and uterine anatomy, to provide information about placental function, and as an especially good predictor of outcome for fetuses that are small for gestational age (see Chapter 13). American Congress of Obstetricians and Gynecologists (ACOG) guidelines support the use of umbilical artery Doppler surveillance in the management of fetal growth restriction. Middle cerebral artery Doppler assessment is used to predict fetal anemia in at-risk pregnancies. Ultrasound also includes three-dimensional (3D) and four-dimensional (4D) technologies. In fetal medicine, the use of these technologies may provide assessment of fetal anomalies of the limbs, thorax, spine, central nervous system, and face. Some centers use 4D ultrasound for guided needle procedures such as amniocentesis and cordocentesis to improve accuracy of the procedure. 3D and 4D ultrasounds are also used to evaluate fetal heart function and structure.
Although magnetic resonance imaging (MRI) is used at some prenatal centers, it is still under investigation for diagnostic evaluation in pregnancy, especially in the final trimester (see Chapter 16). MRI is most often used to define central nervous system defects. Some of the advantages of MRI during pregnancy are that it is a noninvasive technique, it permits easy differentiation between fat and soft tissue, it does not require a full bladder, and it can show the entire fetus in one scan. Currently, MRI confirms fetal abnormalities found by ultrasound and can be used for pelvimetry, placental localization, and determination of size. Fetal MRI is used at medical centers that specialize in fetal diagnosis and treatment (particularly those that perform fetal surgery). Ultrafast MRI is used for evaluation of congenital anomalies that are potentially correctable, such as congenital diaphragmatic hernia, neck masses that result in airway obstruction, myelomeningocele, and cleft lip and cleft palate. MRI is especially useful for definition of maternal anatomy in cases of suspected intra-abdominal or retroperitoneal disease.
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