Prenatal diagnostics: fetoscopy and biochemical screenings

Prenatal diagnosis is a system of testing that helps assess the risk and identify chromosomal and certain structural abnormalities in the fetus, as well as conditions that require adjustments to pregnancy and birth management. An important principle is a step-by-step approach: screening tests are offered first to assess the likelihood, and if the risk is elevated, diagnostic procedures are prescribed to provide confirmation. This approach reduces the number of invasive interventions and optimizes the care of pregnant women. [1]

The current screening methodology includes routine ultrasound examinations and maternal blood tests. In the first trimester, combined screening and noninvasive free placental DNA testing are prioritized, while in the second trimester, anatomical ultrasound and, if necessary, biochemical screening are used. The accepted rule is to select a single, meaningful screening approach rather than combining several different algorithms. [2]

If the screening result increases the likelihood of aneuploidy or significant ultrasound markers are detected, diagnostic tests are offered: chorionic villus sampling at 10-13 weeks or amniocentesis after 15 weeks. These procedures allow for obtaining fetal cells for karyotyping and molecular diagnostics and provide a definitive answer. The decision is made after consultation and risk assessment. [3]

A separate section of prenatal care is fetal therapy. Fetoscopy, as a method of direct endoscopy of the uterine cavity, is currently used primarily for therapeutic purposes, such as twin-to-twin transfusion syndrome in monochorionic twins, and much less frequently for diagnostic purposes. This approach is performed in specialized centers. [4]

Table 1. Screening versus diagnosis: what's the difference?

Criterion	Screening	Diagnostics
Target	Probability assessment	Confirm or exclude
Examples	Combined screening, cfDNA	Chorionic villus biopsy, amniocentesis
Risk to the fetus	Absent	Small but non-zero
Result	Probability	The final answer
When is it prescribed?	To everyone, by choice	In case of increased risk or upon request after consultation

First trimester: combined screening and non-invasive cfDNA testing

Combined first-trimester screening includes ultrasound assessment of nuchal translucency and biochemical markers and is performed at approximately 11-13 weeks. It helps identify an increased risk of Down, Edwards, and Patau syndromes and provides a basis for further steps. Correct dating and adherence to time windows increase the accuracy and value of the results. [5]

Testing for placental cell-free DNA in maternal blood is available from early pregnancy and is the most sensitive and specific screening test for common aneuploidies. However, there remains a risk of false-positive and false-negative results, as well as failures due to a low fetal fraction in some patients, so a positive result requires confirmation by a diagnostic method. [6]

The choice between combined screening and cfDNA depends on availability, timing, clinical factors, and family preferences. A sequential strategy is possible: combined screening initially, followed by cfDNA testing as a clarifying step before invasive diagnosis in cases of intermediate risk. This stepwise approach reduces the number of unnecessary invasive procedures. [7]

An important requirement is informed consent. Patients are explained that screening does not provide a diagnosis, and the possibilities, limitations, and next steps, regardless of outcome, are discussed. This allows for informed decision-making and reduces anxiety. [8]

Table 2. Comparison of first trimester screening

Parameter	Combined screening	cfDNA
Deadlines	11-13 weeks	From 9-10 weeks onwards
Object	Ultrasound and serum markers	Free placental DNA
What is it aimed at?	Trisomies 21, 18, 13	Trisomies 21, 18, 13, sex chromosomes
Accuracy	Good	The highest among screenings
What happens next if the risk is high?	Diagnostic test	Diagnostic test

Second trimester: anatomical ultrasound, quad test, and the role of the "triple test"

The standard of antenatal care is a detailed ultrasound examination of the fetal anatomy at approximately 18-21 weeks. It examines the organs and systems, identifies most major structural anomalies, and determines the location of the placenta. Timely planning of treatment depends on the quality of this examination. [9]

If the first trimester window is missed, biochemical screening may be offered in the second trimester. Historically, the "triple test" was used, measuring alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol. Today, when necessary, the "quad test," which adds inhibin A, is more often preferred. This improves the detection of trisomy 21 compared to the triple test, although both are inferior in accuracy to noninvasive free DNA testing. [10]

The limitations of second-trimester serum testing should be understood: it is a probability assessment, not a diagnosis, and its sensitivity and specificity are lower than those of first-trimester strategies and cfDNA. In cases of increased risk, the algorithm remains the same: consultation and confirmation with the diagnostic intervention of choice. [11]

Serum markers are particularly valuable in providing clues to maternal risks. For example, significant abnormalities in alpha-fetoprotein are associated with neural tube defects in the fetus and certain placentation complications, which increases the need for dynamic monitoring and Doppler ultrasound. Interpretation is performed in conjunction with ultrasound data. [12]

Table 3. Second trimester biochemical screenings: brief

Test	Markers	Comment
Triple	Alpha-fetoprotein, human chorionic gonadotropin, unbound estriol	Historical variant, lower accuracy for trisomy 21
Quadro	Triple Plus Inhibin A	Higher accuracy than triplet, but inferior to cfDNA
cfDNA	Free placental DNA	The most accurate screening, positive results are confirmed diagnostically

Diagnostic tests: chorionic villus sampling and amniocentesis

Chorionic villus sampling (CVS) is typically performed between 10 and 13 weeks of gestation, providing chorionic villi for karyotyping, microarray analysis, and targeted molecular testing. The advantage is early definitive diagnosis. Risks are low, but they exist, so the procedure is performed in experienced centers with mandatory ultrasound monitoring. [13]

Amniocentesis is typically performed after 15 weeks. The cells obtained from the amniotic fluid are used for the same diagnostic purposes. Under current conditions, with proper technique, the risk of pregnancy loss associated with the procedure is low, as confirmed by data from large observational studies and updated reviews. The choice between methods depends on the gestational age, clinical indications, and family preferences after consultation. [14]

Both procedures require an explanation of possible outcomes, expected timeframes for results, and options for further action. If clinically significant changes are identified, a monitoring plan is developed, the location and method of delivery are determined, and early training of neonatologists and related specialists is provided. This approach improves perinatal outcomes. [15]

In multiple pregnancies, the management strategy and risks are discussed separately, taking into account the characteristics of placentation and the need for separate specimen collection. In complex cases, interdisciplinary teams from perinatal centers are involved. [16]

Table 4. Chorionic villus sampling and amniocentesis: comparison

Characteristic	Chorionic villus sampling	Amniocentesis
Material type	Chorionic villi	Amniotic fluid
Normal terms	10-13 weeks	From 15 weeks
Plus	Early final answer	Wide availability, stable terms
Minus	Requires experience, rare specific complications	Waiting up to 15 weeks
Basis of choice	Duration, indications, preferences	Duration, indications, preferences

Fetoscopy: What is it, when is it needed, and what are the risks?

Fetoscopy is an endoscopic approach to the uterine cavity for visualization and procedures on the placenta, umbilical vessels, or fetus. Today, its primary indication is the treatment of twin-to-twin transfusion syndrome in monochorionic twins using fetoscopic laser coagulation of placental vessels. This approach is recognized as the standard for early treatment in advanced stages, as it improves survival and reduces neurological complications compared to expectant management. [17]

The technique and details of the intervention depend on the anatomy of the placenta. There are coagulation options, including selective and the "Solomon technique," aimed at reducing recurrences and associated syndromes. The choice of method is determined by the characteristics of the placental vascular network and the center's experience. [18]

In addition to laser coagulation for twin-to-twin transfusion syndrome, fetoscopy is used in some centers for certain intrauterine surgical procedures, such as the correction of open spina bifida. These cases are subject to strict selection criteria, an emphasis on multidisciplinary support, and clear informed consent. Research is ongoing to compare open and fetoscopic techniques. [19]

Like any invasive procedure involving the fetal membranes, fetoscopy increases the risk of premature rupture of membranes, chorioamnionitis, and preterm labor. Risk-benefit assessment is individualized, and post-procedure monitoring includes frequent checkups and complication prevention according to center protocols. [20]

Table 5. Fetoscopy in twin-to-twin transfusion syndrome

Indication	The essence of the procedure	Expected benefits	Main risks
Monochorionic twins with progressive twin-to-twin transfusion syndrome	Fetoscopic laser coagulation of placental anastomoses	Increased survival, reduction of severe neurological outcomes	Premature rupture of membranes, infection, preterm labor

How to choose a strategy

Choosing a strategy begins with discussing the timing and availability of first-trimester screening. If the timing is met, combined screening or cfDNA testing is appropriate, taking into account clinical factors. If the window is missed, an anatomical ultrasound examination and, if indicated, second-trimester biochemical screening are recommended. It is always important to remember that screening is not the same as diagnosis. [21]

In cases of increased risk or significant ultrasound findings, a diagnostic procedure is offered. The risks of invasive procedures in modern conditions are low, but not zero, so the discussion is based on data, possible scenarios, and the family's values. Consent documentation is mandatory. If severe anomalies are detected, options for routing and perinatal support are considered. [22]

Fetal therapeutic interventions, including fetoscopy, are considered in a limited number of indications at experienced centers. The decision is made collaboratively with perinatologists, neonatologists, anesthesiologists, and psychologists. This format improves safety and the quality of communication. [23]

In all cases, a woman has the right to refuse any element of screening or diagnostic testing. The team's goal is to provide clear, honest, and relevant information without pressure, so that the choice is truly informed and consistent with the family's values. [24]

Table 6.

Step	What to do
1	Discuss goals, timing, and available first trimester screenings
2	Choose one meaningful screening route
3	If there is an increased risk, offer chorionic villus sampling or amniocentesis
4	In case of monochorionic twins and suspected twin-to-twin transfusion syndrome, refer to a fetal therapy center
5	Document consent, plan, and contact information

Answers to frequently asked practical questions

Should I triple-test if I'm planning or have already had cfDNA? No, performing multiple screening algorithms simultaneously does not improve accuracy and may confuse interpretation. A single, coordinated strategy is preferred. [25]

What's the best choice when presenting at 16 weeks without earlier screening? Perform an anatomical ultrasound during the standard window and discuss a quadruple DNA test or cfDNA, taking into account the timing and local protocols. If the risk is high, proceed to diagnostic testing. [26]

How dangerous are chorionic villus sampling and amniocentesis? When performed by an experienced team and under ultrasound guidance, the risk of pregnancy loss is low. The choice of method depends on the gestational age and clinical need. [27]

When is fetoscopy really necessary? When fetal surgery is indicated, primarily for twin-to-twin transfusion syndrome in monochorionic twins. The decision is made in a specialized center after a detailed discussion of the risks and benefits. [28]

Table 7. What to discuss during the consultation before testing

Topic	Why is it important?
Objectives and limitations of the selected screening	Realistic expectations and an action plan
Timing and logistics	Timeliness and accuracy
Threshold for transition to diagnostics	Understanding when a definitive answer is needed
Risks and alternatives to invasive procedures	Informed consent
Contacts and routing	Quick solutions for non-standard results

Conclusions

1. The basis of modern prenatal diagnostics is high-quality ultrasound examination and a well-thought-out choice of one screening route, most often combined first trimester screening or cfDNA. [29]
2. In cases of increased risk or alarming ultrasound findings, diagnostic methods are indicated - chorionic villus sampling or amniocentesis - which provide a definitive answer and allow further planning. [30]
3. Fetoscopy is a fetal surgical tool with limited but vital indications, primarily for twin-to-twin transfusion syndrome in monochorionic twins; it is performed in specialized centers. [31]