Prenatal screening

| Prenatal screening is the set of non-invasive tests that allow us to identify the pregnancies with the highest probability of chromosomal alterations. This probability is called the risk index and is expressed as a fraction (1/270). An index greater than 1/270, for example 1/150, is considered a high risk.

All cells in our body contain genetic information from parents in a substance called DNA. Cells have bundles of DNA in their nucleus, forming chromosomes. All human cells except the sexual cells, which are the ovum and the sperm, have a total of 46 chromosomes grouped in 23 pairs, 23 chromosomes from the father and 23 from the mother. Of these 23 pairs, 22 of them are autosomes, the non-sex chromosomes, called from 1 to 22, and a pair of chromosomes are the sex chromosomes, XX in the case of a female and XY in the case of a male.

Chromosomes can be altered in number or shape. Numerical alterations consist of an abnormality in the number of copies of a chromosome, so instead of having two copies a chromosome can have one (monosomy) or three copies (trisomy). The best-known chromosome disease is Down syndrome, in which there are three copies of chromosome 21 (trisomy 21) instead of two. Other detectable chromosomal abnormalities are trisomy 13 or Patau syndrome, trisomy 18 or Edwards syndrome, and monosomy X or Turner syndrome. Structural abnormalities are those in which a piece of chromosome is altered, it may well be absent or moved (translocation).

The prevalence of fetal chromosomal abnormalities increases proportionally with maternal age. The only way to obtain a certain diagnosis is with invasive procedures2 (amniocentesis, chorionic biopsy…) but these tests always carry a risk of pregnancy loss.

The screening will indicate the risk index of chromosomopathy that each patient has for that specific gestation, it will never make a safety diagnosis of chromosomal alteration. The estimation of the calculation is carried out by means of a computer program that assesses different variables. Biochemical screening is obtained from the result of combining maternal age and biochemical markers, that is, the level of hormones in maternal blood. If we add ultrasound markers to these, we obtain the result of the combined screening, which is the one with the highest rate of abnormality detection, around 85-90% detection.

A few years ago, second trimester screening was used that combined maternal age with the level of two substances in maternal blood, beta HCG and alpha-fetoprotein. This test was carried out between 14 and 16 weeks of gestation.

The first trimester combined screening is currently being carried out. First of all, maternal age is taken into account at the time of delivery. The risk of chromosomal alteration is highly dependent on the age of the pregnant woman, which is estimated to be:

• 1/1200 at age 20
• 1/1050 at age 25
• 1/700 at age 30
• 1/280 at age 35
• 1/131 at age 38
• 1/76 the 40 years
• 1/25 at age 45.

To calculate the biochemical marker, a maternal blood is drawn between 9 and 13 weeks, determining the level of beta HCG and PAPP-A (plasma protein A associated with pregnancy). The level of these hormones varies in mothers carrying fetuses with chromosomal abnormalities, in Down syndrome beta HCG is usually elevated and PAPP-A decreased. In Patau and Edwards syndromes both are usually diminished. Finally, for ultrasound markers, an ultrasound is performed, which can be transvaginal or abdominal, depending on the characteristics of each patient and the preferences of each doctor. First of all, gestational age is determined accurately by measuring the fetus from head to rump, a measure that is often abbreviated as CRL or LCN.

The nuchal fold or nuchal translucency is then measured, which is the accumulation of fluid in the back of the neck, between the spine and the skin of the neck. It is the most important ultrasound marker of the first trimester, which is found increased in more than 70% of fetuses affected by Down, Edwards and Patau syndrome but also in 5% of fetuses without chromosomal alterations, therefore it is never a diagnosis either but an indicator of risk. There are some standards that must be followed for a good measurement of the nuchal fold. An enlarged nuchal fold is considered if it is above the 95th percentile, which according to the CRL of the fetus ranges between 2.5 and 3 mm. The increased nuchal fold is also a marker of possible cardiac malformations of the fetus.

The nasal bone is another ultrasound marker of possible chromosomal alteration since its absence increases the risk of chromosomopathy. The ductus venosus is a blood vessel that carries blood from the umbilical cord to the heart. In the study of the blood flow that passes through the ductus venosus, a flow wave is obtained, which, if altered, also indicates a greater risk of chromosomopathy or congenital cardiac malformation.

Of interest

It is very important to always remember that screening methods are never diagnostic, they simply identify those pregnancies with a high risk of chromosomal alteration so that by performing a minimum number of invasive tests, the largest number of cases of chromosomopathy is detected.

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