A rare de novo prenatal case of Down syndrome due to der(21;21)(q10;q10)

ABSTRACT

Down syndrome (DS) is the most common chromosomal abnormality in the human population, most frequently caused by trisomy 21 due to meiotic nondisjunction. Rarely, DS may result from an isochromosome or a Robertsonian translocation, this being the least common variant. We present a rare prenatal case of Down syndrome caused by a derivative chromosome 21, together with a case-based review of previously reported prenatal der(21;21)/i(21q) rearrangements. We report a case of prenatal diagnosis of DS in a 13-week female fetus, characterized by a derivative chromosome 21, der(21;21)(q10;q10), identified by conventional karyotyping and subsequently confirmed by fluorescence in situ hybridization (FISH). First-trimester ultrasonography showed bilateral jugular lymphatic sacs, marked tricuspid regurgitation, and a single umbilical artery. Quantitative fluorescence polymerase chain reaction (QF-PCR) failed to detect the structural chromosomal abnormality. The rearrangement was considered likely de novo based on normal parental karyotypes. Consequently, the empirical recurrence risk is 1% for de novo cases, whereas in families with a parental 21q;21q rearrangement the recurrence risk may approach 100%. This case highlights the importance of integrating detailed ultrasound screening with molecular and cytogenetic techniques, including QF-PCR, conventional karyotyping, and FISH, for accurate prenatal diagnosis of DS. Precise determination of the underlying chromosomal abnormality is essential for providing accurate genetic counseling, estimating recurrence risk, and facilitating informed reproductive planning for the parents.

1. INTRODUCTION

Down syndrome (DS) is the most common chromosomal abnormality in liveborn infants, with an estimated incidence of approximately 1 in 700 births [1]. It is caused by the presence of an extra copy of chromosome 21 and presents with a wide spectrum of clinical features. While more than 95% of DS cases arise from meiotic nondisjunction, approximately 3–4% are attributable to Robertsonian translocations involving chromosome 21 [2].

The most common Robertsonian translocation associated with Down syndrome is t(14;21), in which a segment of chromosome 21 is attached to chromosome 14, resulting in an unbalanced translocation and a total chromosome count of 46. Approximately 50% of these cases occur de novo, with both parents exhibiting normal karyotypes. In the remaining cases, one parent—most often the mother—is a carrier of the balanced translocation and has 45 chromosomes. The theoretical risk for a carrier mother to have a child with DS is 1 in 3, although the empiric risk is closer to 1 in 10. When the father is the carrier, the risk is lower, approximately 1 in 20.

The second most frequent translocation is t(21;22), in which carrier mothers have an estimated 1 in 10 risk of having an affected child, with a lower risk observed when the father is the carrier.

The rarest form is a homologous Robertsonian translocation, cytogenetically described as der(21;21)(q10;q10). This structural rearrangement results in a functionally complete trisomy 21 and is phenotypically indistinguishable from standard trisomy 21 [3]. The recurrence risk, however, is highly dependent on its origin: if inherited from a parent carrying a balanced 21q;21q translocation, the risk is 100%, whereas de novo cases carry a low recurrence risk [4,5]. Consequently, parental karyotyping is essential in all such cases to provide accurate genetic counseling.

Prenatal identification of Down syndrome has been greatly improved by the integration of detailed fetal malformation scans with genetic testing, including QF-PCR, conventional karyotyping, and FISH. Unlike other forms of DS, the 21q;21q translocation presents unique challenges in genetic counseling, particularly due to the markedly increased recurrence risk when inherited from a parent carrying a balanced translocation [6].

2. MATERIAL AND METHODS

This study presents a focused literature review based on a case report from our practice. We reviewed articles presenting cases with prenatally diagnosed Robertsonian translocation trisomy 21, including articles from Pubmed, Google Scholar and Web of Science databases.

The search combined Medical Subject Headings (MeSH), where applicable, containing the words “de novo”, “prenatal”, “Robertsonian translocation 21”, “Down syndrome”, “homologous translocation 21”, “isochromosome 21”, excluding papers describing mosaic cases, postnatal diagnosis, involving other translocation than t(21;21), or describing cases of balanced carriers. The literature search identified eligible studies published between 2015 and 2025, as seen in Table 1. Reference lists of all eligible articles were manually screened to identify additional relevant studies.

Table 1.

Studies with previously reported prenatal derivative chromosome 21 trisomy.

Study	Gestational age at diagnosis	Karyotype	Detection Method	Parental Origin	Outcome
Hervé et al., 2015 [8]	First trimester (chorionic villus sampling)	i(21q)	Cytogenetics + molecular inference on CVS	Likely maternal (germline mosaicism suspected)	recurrent rob(21;21)
Xu et al., 2020 [11]	Second trimester	der(21;21)(q10;q10)	NIPT + confirmatory cytogenetics	De novo	Down syndrome fetus
Cao et al., 2021 [7]	Second trimester	der (21;21)(q10;q10)	Cytogenetics, MLPA, STR analysis	De novo	Selective fetal termination
Carriero et al., 2025 [9]	Second trimester	i(21)(q10)	Cytogenetics, STR analysis with QF-PCR	De novo	Down syndrome fetus

We also describe a case of a parous 32-year-old Caucasian woman, referred to our clinic for genetic counseling. We tested by QF-PCR fetal and maternal DNA, extracted from CVS and peripheral blood, respectively, with Qiagen blood and tissue kit (https://www.qiagen.com), according to the manufacturer's protocol. The kit used for QF-PCR was Devyser Compact v3 CE-IVD and the data was interpreted in GeneMapper™ Software. We also prepared two different cell cultures from CVS in order to perform classical G-banded chromosomal analysis and FISH test. The FISH kit used was Cytocell Prenatal enumeration Probe.

3. RESULTS

A rare case of monozygotic twins that were discordant for the genetic aneuploidy der (21;21) (q10; q10) has been reported in 2021 by Cao et al., also highlighting the importance of early follow-up with invasive diagnostic genetic testing in order to better manage the case. The parents opted for selective fetal termination by radiofrequency ablation, of the fetus carrying the Robertsonian translocation [7].

A French study from 2015, written by Hervé et al., reports that 90% of homologous translocation trisomy 21 are due to isochromosome 21q, having a risk of recurrence of more than 10%. Confirmatory molecular STR studies are needed in order to diagnose an isochromosome 21q [8].

In a very recent paper, from 2025, Carriero et al. also emphasizes the importance of differentiating between the isochromosome 21q and a Robertsonian translocation, especially for calculating recurrence risk for the couple [9].

Another medical paper points out to the particular biological mechanism these findings can “hide” from molecular tests such as QF-PCR and prenatal screening tests such as non-invasive prenatal tests (NIPTs) [10]. Similarly to our case, Xu et al. [11], report in 2020 a particular Down syndrome case, with a 46,XY,der(21;21)(q10;q10),+21 karyotype, that yielded negative in the screening test, adding this case to the list of 8 prior reported cases by Huijsdens-van Amsterdam et al. [10]. The team states that the reason might be due to biological mechanisms rather than technical limitations or poor quality [10-14].

In prenatal cases the best approach is to combine both cytogenetic and molecular techniques such as FISH, QF-PCR and karyotyping in order to fully characterize the abnormality. It is important to consider that on a classical karyotype there is no difference between a homologous Robertsonian translocation 21q and an isochromosome 21q, further studies are needed such as QF-PCR and other STR differentiation tests [8-17] or centromere specific FISH test.

Once the accurate diagnosis is completed and the mode of inheritance is unraveled, genetic counseling is gold standard in order to discuss management options with the couple. Determining the origin of the translocation—whether de novo or inherited—has critical clinical implications. If a parent carries a balanced 21;21 translocation, the risk of having a child with Down syndrome approaches 100%, as gametes containing the isochromosome cannot yield a balanced genome [4]. Conversely, de novo translocations confer a recurrence risk comparable to that associated with maternal age. Thus, parental karyotyping is indispensable for accurate risk assessment and for guiding reproductive options, including in vitro fertilization with preimplantation genetic testing or the use of donor gametes.

3.1. Case Presentation

A 32-year-old woman, G2P1, was referred for genetic consultation and counseling regarding her ongoing pregnancy showing a first trimester morphology scan with multiple fetal anomalies such as increased nuchal translucency (NT)=2.79mm, hypoplastic nasal bone, bilateral jugular lymphatic sacs, marked tricuspid regurgitation, and a single umbilical artery, as seen in Figure 1 and Figure 2. Based on these findings and the serum screening test, the attending physician estimated the combined risk for Down syndrome at 1 in 40, prompting the decision to perform chorionic villus sampling and prenatal diagnosis to confirm the presence of a potential trisomy 21.

Fig. 1.

Ultrasound showing increased nuchal translucency (NT) – 2,79 mm.

Fig. 2.

Ultrasound showing marked tricuspid regurgitation.

The patient and her husband had no significant personal medical history and reports a non-contributory family history with respect to the fetal diagnosis and they were not consanguineous. The couple has one healthy child, a 5-year-old daughter.

The pregnancy was monitored by the attending obstetrician in accordance with standard care protocols, with all investigations performed promptly and in a timely manner.

A rapid aneuploidy detection method, quantitative fluorescence PCR (QF-PCR), was initially employed after obtaining the patient's informed consent. The result was a normal female profile, with the presence of two X chromosomes and the absence of a Y chromosome, consistent with a female fetus without any chromosomal abnormalities within the tested loci/chromosomes (13/18/21/X/Y), as seen in Figures 3, 4 and 5. QF-PCR kits have a known limitation in detecting structural rearrangements, such as Robertsonian translocations or isochromosomes. QF-PCR was able to exclude maternal contamination.

Fig. 3.

QF-PCR of EDTA blood from mother in parallel with CVS from fetus, showing a normal, false negative result in the fetus.

Fig. 4.

QF-PCR – normal result – first diagram represents maternal EDTA blood and second diagram represents CVS of the fetus – both showing 2 peaks (21H) for chromosome 21.

Fig. 5.

QF-PCR – normal result – first diagram represents maternal EDTA blood and second diagram represents CVS of the fetus – both showing 2 peaks (21C) for chromosome 21.

Given the limitations of QF-PCR in detecting structural chromosomal abnormalities, we continued with classical chromosomal analysis that revealed an abnormal female fetal karyotype, characterized by the presence of a derivative chromosome in all of the 20 analyzed metaphases. This anomaly was attributed to a derivative chromosome 21, Robertsonian translocation between two homologs of chromosome 21, or isochromosome 21q, making the prenatal diagnosis possible, as seen in Figure 6 and Figure 7. To further document the case and confirm the diagnosis, we continued with prenatal FISH from chromosome culture, that also showed three signals of chromosome 21, with metaphase cells showing two of the red signals on the same chromosome, as seen in Figure 8.

Fig. 6.

Female fetus with Down syndrome 46,XX,der(21;21)(q10;q10),+21 – metaphase 1.

Fig. 7.

Female fetus with Down syndrome 46,XX,der(21;21)(q10;q10),+21 – metaphase 2.

FISH technique showing Down syndrome with derivative chromosome 21; Female 46,XX,der(21;21)(q10;q10),+21. ish (D21S270/D21S1440/D21S341)x3.

The complete ISCN 2024 formula for the case was: 46,XX,der(21;21)(q10;q10),+21. ish (D21S270/D21S1440/D21S341)x3.

Due to this particular constellation of the derivative chromosome 21, STR techniques – other than QF-PCR, which was performed two times and yielded the same, false negative results were considered to also fail, making it difficult to differentiate between a true isochromosome and a Robertsonian translocation.

The rarity of this condition is further illustrated by the fact that this was the only such case encountered by the authors during a three-year period in a cytogenetics laboratory processing more than 2,500 cases annually. To our knowledge this is the fifth reported prenatal case of der(21;21)/i(21q) rearrangement.

Parental high-resolution chromosomal analysis was performed, from peripheral blood and 30 cells were analyzed. Both parents were phenotypically and cytogenetically normal, as well as the other child they had, so we assumed that with the exception of ovarian or gonadal mosaicism, the finding in the fetus was de novo.

Based on these results, a genetic diagnosis of de novo trisomy 21 (Down syndrome) secondary to a derivative chromosome 21 was established.

These findings led the couple to elect termination of the current pregnancy and to pursue genetic counseling regarding future reproductive options. The estimated risk of recurrence is low, ranging from 1-10% depending on the nature of the rearrangement [8]. This estimate reflects the uncertainty regarding the exact mechanism (isochromosome versus Robertsonian translocation) and the possibility of parental germline mosaicism. The couple was recommended prenatal screening in future pregnancies, or, depending on the ultrasound, prenatal genetic testing.

4. DISCUSSION

This case underscores the diagnostic and genetic counseling challenges associated with apparently de novo rearrangements in chromosome 21, a rare cytogenetic abnormality with significant implications for prenatal management and family planning. Although phenotypically the fetus presents similarly to classic trisomy 21, the underlying genetic mechanism differs substantially, altering both recurrence risk and available management options [8,9].

The presence of sonographic markers—including bilateral jugular lymphatic sacs, marked tricuspid regurgitation, and a single umbilical artery, increased NT and hypoplastic nasal bone—raised suspicion for fetal aneuploidy, despite a false negative QF-PCR analysis of chorionic villi. This discrepancy highlights the biological mechanisms underlying the formation of derivative chromosomes rather than technical errors or poor quality of the kit. Conventional karyotyping of chorionic villus sampling (CVS), complemented by FISH, was therefore essential to confirm the presence of a derivative chromosome 21.

While QF-PCR provides rapid and highly specific detection of common aneuploidies, it is not designed to identify unbalanced translocations or other structural aneuploidies, potentially generating false-negative results. Accordingly, in pregnancies with high-risk screening results or suggestive ultrasound findings, normal rapid-test outcomes should be interpreted with caution, and conventional karyotyping remains the gold standard [15-18].

All four reviewed studies emphasize the growing role of prenatal ultrasound in the early detection of fetal anomalies, although they differ substantially in their objectives, timing of diagnosis, ultrasound markers investigated, and the novelty of their findings.

Prenatal ultrasound served as the initial indication for further investigation in the studies by Cao et al. [7], Carriero et al. [9], and Xu et al. [11], with most abnormalities being detected during the second trimester. Hervé et al. [8] and our study describe ultrasound anomalies identified in the first trimester of pregnancy. In the report by Cao et al. [7], ultrasound examination of a monochorionic twin pregnancy complicated by twin-to-twin transfusion syndrome identified discordant fetal abnormalities that prompted invasive prenatal diagnosis, ultimately revealing a homologous Robertsonian translocation [der(21;21)] in only one twin. Similarly, Xu et al. [11] described a fetus with ultrasound features suggestive of Down syndrome despite a false-negative non-invasive prenatal testing (NIPT) result, highlighting that sonographic findings remain essential for identifying affected pregnancies even when screening results are reassuring. In contrast, Carriero et al. [9] demonstrated the value of integrating prenatal ultrasound with conventional karyotyping, fluorescence in situ hybridization (FISH), chromosomal microarray analysis, and molecular characterization, providing a more comprehensive diagnostic workflow and improving genotype–phenotype correlation. Unlike these clinically oriented reports, Hervé et al. [8] focused primarily on the cytogenetic origin of homologous chromosome 21 rearrangements and questioned whether apparently de novo rea(21;21) chromosomes are truly de novo, demonstrating that parental gonadal mosaicism may be underestimated and should be considered during genetic counseling. Our case similarly represents a presumed de novo rearrangement in which parental gonadal mosaicism cannot be completely excluded. Regarding the origin of the rearrangements, Cao et al. [7], Carriero et al. [9], and Xu et al. [11] reported de novo homologous chromosome 21 rearrangements, whereas Hervé et al. [8] emphasized that presumed de novo cases require careful parental investigation before excluding inherited gonadal mosaicism. Our case also describes a presumed de novo rearrangement. The principal novelty of each study also differs: Cao et al. [7] reported the exceptionally rare occurrence of discordant homologous Robertsonian translocation trisomy 21 in monozygotic twins affected by twin-to-twin transfusion syndrome; Xu et al. [11] highlighted the diagnostic limitations of NIPT in detecting rare chromosome 21q;21q rearrangements and underscored the continuing importance of ultrasound; Hervé et al. [8] provided new insight into the mechanisms underlying homologous chromosome 21 rearrangements and their implications for recurrence risk; and Carriero et al. [9] expanded prenatal diagnosis through the integration of advanced cytogenetic and molecular techniques. Our case report underlines the diagnostic limitations of QF-PCR as a stand-alone test in prenatal diagnostic. We conclude that in such specific cases, QF-PCR should be complemented by conventional karyotyping and additional molecular techniques when clinically indicated.

Together, these studies demonstrate that accurate prenatal diagnosis of chromosome 21q homologous rearrangements relies on the complementary use of detailed ultrasound assessment, confirmatory invasive genetic testing, and comprehensive molecular characterization, while emphasizing the importance of accurately determining whether the rearrangement is truly de novo for appropriate genetic counseling.

Beyond the medical considerations, prenatal diagnosis via invasive methods introduces ethical and emotional challenges for the couple [19]. Genetic counseling must therefore be empathic, multidisciplinary, and balance comprehensive medical information with psychological support.

This case highlights the importance of a structured diagnostic algorithm integrating ultrasound screening, rapid molecular testing, and conventional cytogenetic analysis, as well as the essential role of personalized genetic counseling. Such an approach enables informed decision-making regarding pregnancy management and anticipates the reproductive needs of affected families.

However further studies containing more patients and families are needed in order to calculate real risks, and our case report needs further tests such as SNP-array, WGS (whole-genome sequencing) or long-read sequencing in order to differentiate between a real Robertsonian translocation and an isochromosome 21q. This was a limitation, as we were not able to perform these tests in our laboratory.

5. CONCLUSIONS

This case underscores the importance of comprehensive prenatal genetic evaluation in pregnancies presenting with abnormal ultrasound findings and elevated combined screening risk. Although the clinical phenotype of Down syndrome resulting from a homologous Robertsonian translocation, or an isochromosome - der(21;21)(q10;q10), is clinically indistinguishable from standard trisomy 21, the underlying cytogenetic mechanism has implications for recurrence risk and genetic counseling. In this case, the translocation was presumed to be de novo.

Parental high-resolution karyotyping and analyzing over 30 metaphases remains essential for accurate risk assessment and informed reproductive planning, however we couldn't exclude germinal mosaicism in the parents. Timely prenatal testing, classical karyotyping, and multidisciplinary counseling are critical for guiding decision-making and addressing the medical, ethical, and emotional challenges faced by affected families.

DECLARATIONS

Institutional Ethical Approval

The study was approved by the Ethical Committee of Synevo Romania. Ethic Committee Name: Ethical Committee of Synevo Romania. Approval Code:1/July 2025. Approval Date: July 2025.

Informed Consent Statement

Written informed consent for publication of anonymized clinical information and images was obtained from the parents.

Funding

There was no funding for this article.