Ultrasound examination (ultrasound)
Ultrasound examination is the most common non-invasive method of prenatal diagnostics. Its physical principle is based on the ability of a sound wave to specifically reflect from the interface between two media that have different tissue densities. The device captures these reflected waves, which allows for the visualization of a detailed image of the developing fetus on a monitor screen.
The procedure is performed at various stages of pregnancy and possesses broad diagnostic capabilities. Aside from simply determining the sex of the unborn child, ultrasound allows the physician to detect structural malformations in a timely manner. In particular, the specialist can assess in detail the correct formation of the limbs, brain structures, kidneys, and many other internal organs of the fetus.
Invasive methods: chorionic villus sampling and amniocentesis
If the ultrasound results raise suspicion, invasive procedures are used for the precise diagnosis of chromosomal and biochemical disorders.
Chorionic villus sampling is scheduled for early pregnancy — from the 9th to the 11th week of gestation. The obtained cellular material is examined in several ways:
- Cytogenetic methods — for the visual detection of chromosomal aberrations.
- Biochemical methods — to determine specific enzyme activity or the precise concentration of metabolic products.
- Molecular genetic methods.
At 14–16 weeks of gestation, amniocentesis is performed, which is a complex procedure for collecting amniotic fluid under strict ultrasound guidance. Its execution technique involves puncture of the amniotic cavity: the needle passes directly through the uterine wall and placental tissues. The main goal of the manipulation is to obtain a sample of amniotic fluid containing exfoliated cells of the developing fetus.
Laboratory processing in amniocentesis
Immediately after collection, the amniotic fluid sample is subjected to centrifugation. This is necessary for the physical separation of the material into two fractions: a transparent supernatant and a dense cell pellet. Each part is examined differently.
The supernatant is sent for standard biochemical analysis. The resulting pellet, which represents the fetal cells, is processed according to two scenarios:
- Carrying out a rapid, direct biochemical analysis of the isolated cells.
- Performing cultivation — creating a full-fledged culture of fetal cells in vitro. The cultured material is subsequently used for precise chromosomal analysis (karyotyping) and expanded biochemical testing.
Alpha-fetoprotein (AFP)
The key biochemical marker determined in prenatal diagnostics is alpha-fetoprotein. Structurally, it is a glycoprotein that is actively produced by the fetal liver.
Deviations of AFP levels from the norm have tremendous diagnostic significance:
- Elevated levels often indicate severe developmental anomalies (e.g., anencephaly — congenital absence of the brain) or may indicate intrauterine fetal demise.
- Decreased levels are an alarming marker of major chromosomal pathologies, such as Down syndrome, Edwards syndrome, and a number of other genetic disorders.
Briefly: answers to topical questions
What structural developmental defects of the fetus can be detected using ultrasound?
Ultrasound can detect malformations of the limbs, brain, kidneys, and other organs.
In the 1st trimester (11–14 weeks), ultrasound allows visualization of major malformations:
- CNS malformations: anencephaly, meningocele, voluminous spinal hernias;
- organ agenesis;
- major skeletal anomalies.
In the 2nd trimester (18–21 weeks), ultrasound allows the diagnosis of almost all fetal developmental anomalies.
Through which anatomical structures does the needle pass during amniocentesis?
During amniocentesis, sources describe the puncture of the amniotic cavity through the uterine wall and placenta. The procedure is performed under ultrasound guidance to collect amniotic fluid containing fetal cells.
In which pathologies is an increase in the level of alpha-fetoprotein in the pregnant woman's blood observed?
An increase in AFP levels is observed in fetal developmental anomalies, particularly in anencephaly — the absence of the brain, as well as in intrauterine fetal demise. AFP is a glycoprotein produced by the fetal liver.
A decrease in the level of alpha-fetoprotein is a marker of which chromosomal syndromes?
A decrease in AFP levels is a marker of fetal chromosomal pathologies, including Down syndrome and Edwards syndrome.
What are the absolute and relative contraindications to performing amniocentesis?
Sources do not divide contraindications into absolute and relative, but a general list of conditions in which the procedure is contraindicated is highlighted:
- Infectious diseases — of any localization;
- Inflammatory diseases — of any localization;
- Threatened miscarriage;
- Preterm birth (PTB);
- Rupture of fetal membranes.
In addition, if the pregnant woman has rhesus sensitization, the potential benefits and risks of amniocentesis must be carefully weighed.
What biochemical markers, other than AFP, are included in the first trimester dual screening test?
I trimester biochemical screening includes:
- free β-hCG;
- PAPP-A.
In sources, I trimester screening is performed at 11⁰–13⁶ weeks or 11–14 weeks and includes a combination of ultrasound with blood biochemistry.
Class and exam questions
What is the principle of ultrasound examination?
Ultrasound is based on the ability of the ultrasonic wave to reflect from the boundary between two media with different densities. The reflected signals captured by the device form the final image on the screen.
What does a decreased alpha-fetoprotein level indicate?
A decrease in AFP concentration primarily indicates a high risk of fetal chromosomal pathologies, such as Down syndrome or Edwards syndrome.
Why is the fluid centrifuged during amniocentesis?
Centrifugation makes it possible to separate the sample into a liquid fraction for biochemistry and a cell pellet, from which a cell culture can be grown for detailed chromosomal analysis (karyotyping).
What else to study on this topic
- Cytogenetic methods for detecting chromosomal aberrations
- Determination of enzyme activity in chorionic villus sampling
- Technique of in vitro fetal cell culture
- Cordocentesis as a separate diagnostic method
- Mechanisms of glycoprotein synthesis by the fetal liver
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