Prenatal diagnosis and screening are intended to inform pregnant women and their partners about their fetus’s risk of birth defects or genetic disorders, as well as provide them with advice on how to deal with the uncertainty. Many families that are known to be at high risk of having a child with a major birth defect choose not to have children. Prenatal screening enables women to become pregnant while aware of the fact that the diagnosis will reveal whether or not the fetus has a problem.
Prenatal diagnosis refers to the process of identifying a fetus’ high risk for a genetic abnormality and determining whether or not the fetus is affected by the disorder. The elevated fetal abnormalities is generally found as a result of the birth of a previous child with the illness, a family history of the disorder, a positive parental carrier test, or a positive prenatal screening test. For prenatal diagnosis, an invasive method such as chorionic villus sampling (CVS) or amniocentesis may be necessary to collect fetal cells or amniotic fluid for testing.
Prenatal screening, on the other hand, is frequently used to test for other particular birth disorders, such as chromosomal aneuploidies, neural tube defects, and other developmental anomalies in pregnancy that are not known to raise the risk of heart defects or genetic disorders. Birth defects occur more commonly in newborns who are not regarded to be at high risk, and their parents are not offered prenatal screening. Screening tests are often non-invasive and rely on collecting a sample of maternal blood or imaging, such as ultrasonography or MRI. Screening tests are often designed to be inexpensive and simple enough to be suitable for screening all pregnant women in a population, regardless of their vulnerability.
Methods of Prenatal Diagnosis
Invasive Testing
Amniocentesis
Invasive testing makes use of CVS or amniocentesis to get fetal samples. Amniocentesis is the procedure of inserting a needle through an amniotic membrane and obtaining a sample of amniotic fluid transabdominally. The amniotic fluid contains fetal cells and is further used for diagnostic tests. Ultrasonographic screening is used prior to amniocentesis to determine fetal viability, gestational age (determining various biometric parameters such as head diameter, abdominal circumference, and femur length), number of fetuses, volume of amniotic fluid, normality of fetal anatomical structures, and location of the fetus and placenta to allow an optimal position of the needle within the fetus.
Amniocentesis is usually performed as an invasive procedure between the 16th and 20th week, of gestational age. In addition to fetal chromosome and genome studies, the quantity of alpha-fetoprotein (AFP) in amniotic fluid can be measured to diagnose open neural tube abnormalities (NTDs). AFP is a fetal glycoprotein that is generated mostly in the liver, released into the fetal circulation, and eliminated into the amniotic fluid via the kidneys. The maternal bloodstream transports AFP across the placenta, amniotic membranes, and maternal-fetal circulation. As a result, it can be examined in (amniotic fluid AFP [AFAFP]) or in (maternal serum AFP [MSAFP]).
Chorionic Villus Sampling
CVS needs a transcervical or transabdominal biopsy of chorion villi tissue, which is commonly performed between the 10th and 13th weeks of pregnancy. Chorionic villi are removed from the trophoblast, the extraembryonic component of the blastocyst, and are a suitable source for fetal tissue biopsy. CVS, like amniocentesis, employs ultrasonographic scanning to select the optimum sampling technique.
The main advantage of CVS over mid-trimester amniocentesis is that the results are known at an earlier stage of birth, decreasing the period of doubt and allowing termination in the first trimester if desired. However, unlike amniocentesis, AF-AFP cannot be tested at this stage. As a result, testing for a possible open NTD must be done using alternative methods, such as MS-AFP screening, AF-AFP amniocentesis, and ultrasonography.
Preimplantation Genetic Diagnosis
Preimplantation genetic diagnosis (PGD), also called Preimlantation Genetic Testing (PGT) or Preimplantation Genetic Screening (PGS), refers to analysis of in vitro fertilization (IVF) embryos to detect embryos free of genetic disorders before transferring to the uterus. This procedure was developed in an attempt to provide an alternative to the abortion option for some couples who are at high risk of certain genetic disorders or chromosomal aneuploidy in their babies. PGT would allow them to have a pregnancy and limit abortion . NGS-based techniques are commonly used to detect chromosomal abnormalities in those embryos.
While PGD has been performed hundreds of times using blastomer biopsy all over the world, it is not without controversy. First, molecular analysis of a single cell is technologically demanding; accuracy ranges with false-positive rates of approximately 6% and false-negative rates of around 1%, which is significantly higher than for CVS or amniocentesis specimen evaluation. However, the method for blastocyst biopsy generates more cellular material and appears to be more precise.
Noninvasive Prenatal Diagnosis
Prenatal Diagnosis of Anomalies by Ultrasonography
Chromosomal aneuploidy is associated with a variety of fetal anomalies detected by ultrasound testing, including trisomy 21, trisomy 18, trisomy 13, 45,X and several other rare karyotypes. These anomalies can also occur in a chromosomally normal fetus as isolated findings. The likelihood of a chromosomally abnormal fetus increases dramatically when only one of many abnormalities is a fetal abnormality detected by ultrasonic examination.
Diagnostic ultrasonography can be useful for prenatal diagnosis of some single-gene conditions for which DNA testing is possible because a blood or tissue sample is impractical for DNA or biochemical research. Ultrasonography may also be helpful where the exposure of a genetic disease is unknown and there is no conclusive DNA-based research.
Ultrasonography may also recognize a variety of individual anomalies which can recur in families and which are considered to have multifactorial inheritance, including neural tube malformations. Often available in several clinics is prenatal echocardiography for a comprehensive evaluation of births at risk for congenital heart disease.
Ultrasound testing can be used as early as 13 weeks ‘ gestation to determine fetal sex. For certain women found to be at elevated risk, this assessment can be an significant prelude or supplement in the prenatal treatment of such X-linked recessive disorders (e.g., hemophilia)
PRENATAL SCREENING
Prenatal screening has traditionally relied both on ultrasonography and on the measurement of different proteins and hormones (called analytes) whose maternal serum levels are altered when a fetus is affected by a trisomy or an NTD.
Screening for Neural Tube Defects
The concentration of AFP in maternal serum is likely to be higher than normal when the fetus has an open NTD, just as we had seen in amniotic fluid before. This discovery forms the basis for using the 16-week Maternal Serum AFP (MSAFP) assessment as a screening tool for open NTDs. There is a substantial difference between the standard MSAFP distribution and the spectrum of concentrations observed when an active NTD is present in the fetus. While an elevated concentration of MSAFP is by no means unique to a pregnancy with an open NTD, fetal ultrasonography can discern many of the other causes of elevated MSAFP concentration from open NTDs.
Also, MSAFP is not perfectly sensitive, since its evaluation depends on statistically defined cutoff values. If an elevated concentration in pregnancies is characterized as two multiples of the median value without any abnormality that may raise the concentration of AFP, it can be calculated that 20 percent of fetuses with open NTDs remain undetected. Lowering the threshold to increase sensitivity, however, would come at the cost of decreased accuracy, thereby increasing the risk of false positivity.
Screening for Down Syndrome and Other Aneuploidies
About 70 percent of all children with severe autosomal trisomies are born to mothers who lack known risk factors, including advanced maternal age. A solution to this question was first proposed by the surprising finding that concentration of MSAFP was depressed in several.
First-Trimester Screening (Double Marker Screening)
First-trimester screening is typically conducted between 11 and 13 weeks of gestation and relies on the assessment of the amount of certain analytes in maternal serum combined with a tightly selective ultrasonographic test. The analytes used are plasma protein A (PAPP-A) linked with pregnancy and the hormone human chorionic gonadotropin ( hCG), either as a complete hCG or as its free β subunit. In all trisomies PAPP-A is depressed below the normal range; hCG (or free β-hCG) is elevated in trisomy 21 but depressed in other trisomies. Analyte measurements are paired with ultrasonographic measurement of nuchal translucency (NT), defined as the thickness of the echo-free area between the skin and the soft tissue overlying the dorsal part of the cervical spine caused by the fetal neck’s subcutaneous edema. An rise in NT is usually found in 21, 13, and 18 trisomies and in 45,X fetuses. Since NT varies with fetal age, it must be evaluated in relation to gestational age.
First Trimester Screening
| Nuchal Translucency | PAPP-A | Beta-HCG | |
| Trisomy 21 | ↑ | ↓ | ↑ |
| Trisomy 18 | ↑ | ↓ | ↓ |
| Trisomy 13 | ↑ | ↓ | ↓ |
Second-Trimester Screening (Triple Marker and Quadruple Markers)
Second-trimester screening is usually conducted by evaluating hCG in conjunction with three other analytes: MSAFP, unconjugated estriol(uE3), and inhibin A. The test panel is called a quadruple panel. Both of these compounds in all trisomies with the exception of hCG, which is elevated in trisomy 21 but depressed in the other trisomies, and inhibin A, which is elevated in trisomy 21 but not greatly impaired in the other trisomies, are depressed below normal. There are a variety of factors that can influence rates of these analytes, including age, alcohol, IVF pregnancy, and maternal diabetes, and laboratories that typically compensate for these variables. Extremely low levels of unconjugated estriol may indicate rare genetic conditions, such as deficiency in steroid sulfatase or Smith-lemli-Opitz syndrome.
Second Trimester Screening
| uE3 | AFP | Beta-HCG | Inhibin A | |
| Trisomy 21 | ↓ | ↓ | ↑ | ↑ |
| Trisomy 18 | ↓ | ↓ | ↓ | – |
| Trisomy 13 | ↓ | ↓ | ↓ | – |
| NTD | – | ↑↑ | – | – |
Noninvasive Prenatal Screening by Analysis of Cell-Free Fetal DNA (cffDNA)
The area of prenatal screening and obstetrical genetics is being revolutionized by the convergence of two significant developments in genomics, one biological and the other computational, to develop a modern prenatal screening technique known as non-invasive prenatal screening (NIPS) (sometimes referred to as non-invasive prenatal research, NIPT). The scientific finding is that after 7 weeks of birth, a pregnant woman ‘s blood incorporates fetal DNA, which is not found in a cell’s nucleus but flows naturally in the maternal bloodstream. Approximately 2 to 10% of the non-cellular DNA in maternal blood is derived from placental trophoblasts and is therefore of fetal origin. While combined with maternal DNA, this cell-free fetal DNA offers a snapshot of the fetal genome that is suitable for study without the need for an intrusive technique.The technological breakthrough is the development of high-throughput sequencing methods which allow millions of individual DNA molecules to be sequenced into a mixture.
NIPT allows for highly precise, noninvasive monitoring of pregnancies for common autosomal and sex chromosome aneuploidies, with sensitivities and specificities near 99 percent for trisomy 21. Cell-free fetal DNA in maternal serum has also been used to genotype the fetus at the Rh locus and determine fetal sex. Further advancements in cell-free DNA research will make noninvasive testing for many other genetic diseases, including many single-gene disorders, eligible for clinical treatment in the future.
Non-invasive prenatal monitoring based on cffDNA using next-generation sequencing (NGS) is a highly sensitive and precise method for fetal aneuploidy screening. NIPT employs a variety of genetic tools, including microarray-guided sequencing and whole-genome sequencing. Since the advent of NGS technology, NIPT has also been used in many sequencing systems, including a sequencing semi-conductor platform (Ion torrent sequencing) and the Illumina sequencing platform. Despite being built on separate sequencing concepts, both the Ion Torrent and Illumina sequencing systems performed admirably in detecting T13, T18, and T21.
References:
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- Cogulu O. Next Generation Sequencing as a Tool for Noninvasive Prenatal Tests. In: Clinical Applications for Next-Generation Sequencing. Elsevier Inc.; 2016:171-188. doi:10.1016/B978-0-12-801739-5.00009-X
- Curnow KJ, Sanderson RK, Beruti S. Noninvasive Detection of Fetal Aneuploidy Using next Generation Sequencing. Vol 1885. Second. (Levy B, ed.). New York, NY: Springer New York; 2019. doi:10.1007/978-1-4939-8889-1_22
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