**3. Methods of prenatal screening and diagnosis**

The main procedures of prenatal screening are biochemical screening and ultrasound that allow the identification of pregnancies with increased genetic risk. The prenatal screening methods were introduced in medical practice in the1980s, based on the association between incidence of trisomy 21 (Down syndrome) and advanced maternal age. A decade later both the maternal serum biochemistry and detailed ultra-sonographic examination in the second trimester were developed and allowed the identification of high-risk pregnancies. In the 1990s, the prenatal screening shifted to first trimester by a combination of maternal age, fetal nuchal translucency (NT) thickness and maternal serum-free β-hCG and PAPP-A. In the last 10 years, the prenatal screening methods changed again by the introduction of genomic screening that searches free fetal DNA in maternal blood [8].

The prenatal diagnosis imposes the obtaining of embryonic or fetal cells by using invasive methods like chorionic villus sampling (CVS), amniocentesis or cordocentesis. All these methods present risks for spontaneous miscarriages, obstetrical hemorrhages and fetal damages. The embrionar or fetal cells are used for cytogenetic or molecular diagnosis.

#### **3.1. Biochemical screening**

Triploidy is unrelated to maternal age and the prevalence is about 1/2000 at 12 weeks, but the

**Figure 6.** Trisomy 13 (collection of cytogenetic laboratory, "Grigore T. Popa" University of Medicine and Pharmacy Iaşi,

**Figure 5.** Trisomy 18 (collection of cytogenetic laboratory, "Grigore T. Popa" University of Medicine and Pharmacy Iaşi,

Romania).

370 Congenital Anomalies - From the Embryo to the Neonate

Romania).

The birth of a child is an important event in the life of every family and represents the end of a long period of uncertainty generated by fear that the future child will be abnormal. The high incidence of chromosomal anomalies during prenatal life and the severity of phenotype of chromosomal anomalies that allow survival, imposed the development of methods of prenatal

majority of affected fetuses die by 20 weeks and the born babies are mosaics 46/69 [7].

Biochemical screening is based on the determination of maternal serum markers that are associated with an increased risk of chromosomal diseases. Biochemical screening could be applied during the first or the second trimester of pregnancy. The presence of a fetal aneuploidy is associated with changes of maternal serum concentrations of some fetoplacental products: aFP, free ß-hCG, inhibin A, μE3 and PAPP-A. In normal pregnancies, aFP concentration in maternal serum increases from 11.3 ng/ml in the 8th week of gestation to 250 ng/ml in the 32nd week of pregnancy. After that, it then declines slightly until term. In normal pregnancies, hCG in the maternal serum increases in first trimester of pregnancy and reaches a peak in weeks 7–9 (100.000 IU/ml). After that, it reduces continuously until around 20 weeks of pregnancies, when reduction is stopped and plasma levels remain constant until term. In normal pregnancies, inhibin A increases from the 6th week of pregnancy to the 9th week of pregnancy and reaches a peak (~550 pg/ml). After that, the values decline continuously to the 14th week of pregnancy. In normal pregnancies, μE3 is first detectable at 9 weeks of gestation (0.05 ng/ml) and after that it increases continuously to about 30 ng/ml at term. In normal pregnancies, PAPP-A is first detected in maternal serum after 4 weeks of pregnancies. PAPP-A concentration increases exponentially in the first trimester. After that, the rise occurs slowly, but it continues until delivery [9–12].

*3.1.1.3. Other biochemical markers*

ing in the first trimester of pregnancy [18, 19].

*3.1.1.4. Screening of other aneuploidies*

*3.1.2. Second-trimester prenatal screening*

for free ß-hCG than total hCG [24–27].

*3.1.2.1. Alpha-fetoprotein*

gonadotropin, unconjugated estriol and inhibin-A.

the detection of 55% of cases with trisomy 21 [28, 29].

trimester screening [14].

The serum marker used in second-trimester screening (aFP, μE3 and inhibin-A [INH-A]) shows minimal differences in trisomy 21 pregnancies and thus could not be used in first-

Prenatal Biochemical and Ultrasound Markers in Chromosomal Anomalies

http://dx.doi.org/10.5772/intechopen.73604

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ADAM 12 is a glycoprotein synthesized by the placenta and secreted through pregnancy. ADAM 12 presents proteolytic functions and has a low level in first-trimester cases with trisomy 21 or trisomy 18. This reduction is more pronounced in earlier gestation, with best results at around 8–10 weeks. ADAM 12 in combination with PAPP-A (both measured at 8–9 weeks), nuchal translucency (NT) and free β-hCG measured at 12 weeks allow a detection rate of 97% at a 5% false positive rate and thus could be the best protocol for prenatal screen-

The screening of other aneuploidies in the first trimester of pregnancy is also possible and has a good rate of detection. Low maternal serum PAPP-A was identified in trisomy 18, trisomy 13, triploidy and monosomy X. Low levels of free β-hCG were discovered in trisomy 18, trisomy 13 and in some cases of triploidy [20–23]. However, all these disorders have a high rate of spontaneous miscarriages (minimum 80%) and any conversion of the observed detection rates to true detection rates is, therefore, associated with a substantial degree of uncertainty [13].

Second-trimester serum markers are represented by alpha-fetoprotein, human chorionic

Screening for aneuploidies was initially focused on the second trimester of pregnancy and demonstrated a substantial improvement in detection rates of trisomy 21, compared with screening using only maternal age. At a false positive rate of 5%, the detection rate improves from 30% in screening by maternal age alone to 60–65% by combining maternal age with serum AFP and free ß-hCG (double test), 65–70% with the addition of μE (triple test) and 70–75% with the addition of inhibin A (quadruple test). In the case of hCG, it is better to search

The first report concerning the association between low level of maternal serum alpha-fetoprotein and fetal trisomy 21 was made in 1984, by Merkatz et al. [9]. At a risk cut-off of 1:270 for trisomy 21 (equivalent to the maternal age of 35), using this parameter alone would allow

The aFP is produced by fetal liver, but its biological functions and the reason why the aFP level is lower in Down syndrome pregnancies remain unclear. Placentas of affected pregnancies show a high level of aFP suggesting a defect in the secretion of AFP into the maternal circulation [30].

In biochemical screening, the measured concentration of the markers is converted into a multiple of the median (MoM) of unaffected pregnancies at the same gestation. The Gaussian distributions of log10 (MoM) in trisomy 21 and unaffected pregnancies are then derived, and the ratio of the heights of the distributions at a particular MoM represents the likelihood ratio for a trisomy [7].
