Discordance for Defects in Monochorionic Twins: Prevalence and Impact on Perinatal Outcomes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. Overall reasonable analysis of a rare group.
2. In results why do you report subsequently CNS requires discussion.
3. Table 3 and the hypothesis on how monochorionic creation / splitting of the placenta creates vascular disruptive anomalies of the CNS , face, cardiac and gut/abdominal wall; the comment re CNS and facial and impact on swallowing requires a comparison of the diamniotic sac volumes compared to the anomaly differences. 

Author Response

1. Overall reasonable analysis of a rare group.
2. In results why do you report subsequently CNS requires discussion.
3. Table 3 and the hypothesis on how monochorionic creation / splitting of the placenta creates vascular disruptive anomalies of the CNS , face, cardiac and gut/abdominal wall; the comment re CNS and facial and impact on swallowing requires a comparison of the diamniotic sac volumes compared to the anomaly differences. 

Response: We thank the reviewer for this insightful comment. Quantitative comparison of diamniotic sac volumes was not available in this retrospective dataset and was therefore not performed. We agree that such an analysis would be valuable to further explore the relationship between CNS and facial anomalies, swallowing impairment, and amniotic fluid dynamics. We have revised the manuscript to clarify that this proposed mechanism is hypothetical and to acknowledge this limitation.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is interesting and addresses a rare cohort, but it requires major revisions.

Summary of the study
This is a retrospective study of 21 monochorionic twin pregnancies with discordant structural defects and normal karyotype/aCGH in both fetuses, nested within a cohort of 381 MC twin pregnancies assessed in the first trimester.
The authors describe the types of defects, their prevalence, and their association with single/double fetal death and preterm birth before 32 weeks, emphasizing the role of CRL discordance >20% and polyhydramnios.
They conclude that cardiac defects are the most frequent and that size discordance and polyhydramnios, rather than the type of defect, are associated with adverse perinatal outcomes.

Main strengths
The topic is clinically relevant and of high interest for fetal medicine and obstetrics, focusing on an uncommon condition (MC twins discordant for structural defects with normal aCGH).
The design (multicenter series from three fetal medicine units) includes a clearly defined reference cohort (381 MC pregnancies) and explicit inclusion criteria (structural discordance, normal aCGH, exclusion of severe TTTS treated with laser).
The key messages are clear:

• An approximate prevalence of 5% of discordant structural defects with normal aCGH in MC twin pregnancies.
• CRL discordance >20% is associated with an increased risk of single/double fetal death.
• Polyhydramnios is associated with preterm birth <32 weeks, particularly in CNS and facial defects.
  Tables 1 and 2 provide case-by-case details (type of defect, CRL, complications, interventions and gestational age at delivery), which enhances reproducibility and clinical usefulness.
  The bibliography is adequate, with references to key classical studies on twinning, malformation risk and perinatal outcomes (Nicolaides, Best, Deprest, etc.).

Weaknesses

1. Novelty, scope and significance
   The novelty is suggested (focus on discordant MC twins with normal aCGH managed expectantly), but the text does not specify clearly enough

• What this study adds beyond previous series of discordant twins or MC twins with malformations; the authors should explicitly highlight the specific contributions compared with earlier reports.
• How restricting the cohort to cases with normal aCGH modifies the clinical interpretation in relation to existing literature.
  The research question is somewhat diffuse; an explicit main hypothesis or primary objective would be advisable.

1. Quality of writing and structure
   The English requires a thorough professional revision (grammar, agreement, prepositions, and repeated use of formulaic phrases from FMF/ISUOG guidelines), which currently undermines overall clarity.
   There are multiple typographical and formatting errors:

• Percentages are sometimes reported without the “%” symbol, and spacing and punctuation are inconsistent.
• Inconsistent terminology (CHD/CDH, GUT vs genitourinary defect), which may confuse readers.
• Statistical notation such as “P0.046” instead of “p = 0.046”.
  The overall structure (Abstract – Introduction – Methods – Results – Discussion – Conclusions) is broadly adequate.
• The Abstract is long, has dense sentences, and does not fully follow the structured format typically used in Diagnostics (Background, Methods, Results, Conclusions in concise form).
• The Discussion lacks clearly delineated subsections (main findings, comparison with the literature, clinical implications, limitations, future directions), although the content is present.

1. Methodological robustness and transparency
   Design:

• The study is described as a multicenter retrospective series (2019–2022), but further detail is needed on how cases were identified (ultrasound codes, perinatal registries, etc.) and whether there were any losses to follow-up or additional exclusions.

Inclusion/exclusion criteria:

• Cases with abnormal aCGH and severe TTTS requiring laser therapy are excluded, but it is not specified whether all malformed fetuses underwent aCGH or whether any parents declined invasive testing; this may introduce selection bias.
• It is not clearly stated whether minor anomalies or variants of normal (e.g. small muscular VSD) were excluded.

Methods:

• Important methodological details are missing, including:
• The operational definition of “CRL discordance >20%” (and whether a specific formula was used).
• The definition of sFGR (type I/II/III, EFW <10th centile plus discordance, or based on CRL/EFW).
• Criteria for performing amniodrainage and for iatrogenic induction/caesarean delivery before 32 weeks.
• The statistical section does not specify which software was used or whether any adjustment for multiple comparisons was performed; only the tests (chi-square/Fisher/Mann–Whitney) and a significance threshold of p <0.05 are listed.

Reproducibility:

• Despite informative tables, the text would benefit from:
• Explicit denominators for all percentages (sometimes “47%” is reported without repeating the corresponding N).
• Confidence intervals for key estimates (5.5% prevalence, risk of FD with CRL >20%, risk of preterm birth with polyhydramnios), as is recommended for clinical studies.

1. Figures, tables and data presentation
   Tables 1–3 are useful, but:

• A clearer, fully annotated flow diagram is needed to show the initial cohort, exclusions due to abnormal aCGH and TTTS, and the final number of pregnancies included.
• In Table 3, p-values are given without confidence intervals and without clearly specifying which test was used in each comparison.
  No ultrasound images or illustrative examples of the defects are provided, which could significantly enrich a highly clinical article in Diagnostics, provided ethical and confidentiality requirements are met.

1. Ethical aspects and transparency
   An ethics committee statement with date and decision number is included.
   In the Acknowledgments, a template sentence regarding GenAI use is left in its generic form; MDPI requires concrete disclosure when AI tools are used, and if they were not, this sentence should be removed.
2. Adherence to editorial guidelines
   The literature review is substantial but could be strengthened by including additional references from the last 3–5 years on structural discordance in MC twins or selective fetocide, in line with MDPI’s emphasis on recent citations.
   The “Implications for clinical practice” section proposes strong recommendations (e.g. considering early selective fetocide in cases with polyhydramnios to reduce prematurity) that:

• Are not fully supported by this series (no selective fetocide was performed) but rather by the broader literature.
  These recommendations should be more cautiously framed and more explicitly linked to either the study results or external evidence.

Specific points for improvement

1. Abstract and objectives

• Rewrite the Abstract in a clearly structured format (Background, Methods, Results, Conclusions) including:
• A concise primary objective.
• Quantitative results with denominators and, if possible, confidence intervals.
• Brief conclusions without excessive interpretation.
• In the Introduction, define an explicit hypothesis, for example: “In MC twins discordant for structural defects with normal aCGH, CRL discordance and polyhydramnios are independently associated with fetal death and preterm birth.

1. Methods

• Add methodological details:
• Case ascertainment criteria in the databases of the three centers and whether any cases were missed.
• Precise definitions of CRL discordance and sFGR, with references to ISUOG/FIGO guidelines if applicable.
• Criteria for offering amniodrainage and timing/indication of iatrogenic delivery.
• Statistical software, handling of multiple testing, and presentation of 95% CIs for key variables.

1. Results

• Standardize the presentation:
• Always report N and % together for each proportion (e.g. “preterm birth occurred in 9/19 pregnancies, 47%”).
• Check the internal consistency of numbers between text and tables (particularly for FD and prematurity rates).
• Consider an additional descriptive analysis:
• Compare outcomes by type of defect (cardiac vs non-cardiac) and by amnionicity (MCDA vs MCMA), explicitly acknowledging the limitations imposed by the small sample size.

1. Discussion and conclusions

• Restructure the Discussion along standard lines:
• First paragraph: main findings.
• Second/third paragraphs: comparison with previous studies, emphasizing the added value of restricting the cohort to cases with normal aCGH.
• A specific paragraph on clinical implications, with more nuanced recommendations (e.g. cautious use of amniodrainage, carefully selected indications for selective fetocide).
• A clear paragraph on limitations (retrospective design, small sample size, absence of multivariable analyses, potential selection bias).
• Adjust the Conclusions so that they remain strictly supported by the data, avoiding overstatements (for instance, not proposing changes in clinical practice that are not directly substantiated by the series).

1. Language, format and MDPI style

• Submit the manuscript to a professional scientific English editing service or MDPI’s own editing support, with special attention to:
• Grammar, verb tenses, prepositions, and pluralization.
• Consistency of terminology (MCDA/MCMA, CHD, sFGR, FD, PTB).
• Correct statistical notation and units:
• Use “p = 0.046” instead of “P0.046” and ensure systematic use of “%”.
• Complete or remove the template GenAI sentence in the Acknowledgments according to actual use; MDPI requires explicit disclosure if such tools were employed.

1. Ethical and transparency statements

• Simplify and standardize the following sections in line with MDPI templates:
• “Institutional Review Board Statement”: concise, standard wording.
• “Informed Consent Statement”: clarify whether consent was obtained or waived by the ethics committee.
• “Data Availability Statement”: specify more precisely what data are available and under which conditions.

Author Response

Weaknesses

1. Novelty, scope and significance
   The novelty is suggested (focus on discordant MC twins with normal aCGH managed expectantly), but the text does not specify clearly enough

• Response: Our study provides a focused analysis of structurally discordant monochorionic twin pregnancies with normal aCGH, allowing assessment of outcomes independent of chromosomal abnormalities, which has not been addressed in previous heterogeneous series. We demonstrate that intertwin growth discordance >20% and polyhydramnios—particularly in CNS and facial defects—are stronger predictors of fetal demise and very preterm birth than the type of malformation itself. These findings translate into concrete clinical implications, including the need for routine fetal echocardiography in all monochorionic twins and consideration of early intervention in high-risk cases. Moreover, our study shows that genetically identical monochorionic twins may still present with different structural abnormalities, highlighting the importance of accurate CRL assessment for early detection

 

• What this study adds beyond previous series of discordant twins or MC twins with malformations; the authors should explicitly highlight the specific contributions compared with earlier reports.

• Response: Our study adds to the limited data on structurally discordant monochorionic twins managed expectantly by demonstrating that fetal loss before 24 weeks (25%) is primarily associated with large intertwin discordance (>20%), rather than with the type of malformation. A major strength is comprehensive invasive testing in both twins, including karyotype and aCGH, enabling detection of submicroscopic genomic variants and providing added value over previous series, what distinguishes our study from existing literaturę.

 

• How restricting the cohort to cases with normal aCGH modifies the clinical interpretation in relation to existing literature.
  The research question is somewhat diffuse; an explicit main hypothesis or primary objective would be advisable.

• Response: Our study hypothese that discordance for structural defects in monochorionic twins with normal aCGH occurs frequently. The impact on perinatal outcomes including the rate of single and double fetal loss before 24 weeks’ gestation and the rate of preterm birth before 32 weeks gestation was examined in our study, which remains crucial in manage pregnancy.

The English requires a thorough professional revision (grammar, agreement, prepositions, and repeated use of formulaic phrases from FMF/ISUOG guidelines), which currently undermines overall clarity.
There are multiple typographical and formatting errors:

• Percentages are sometimes reported without the “%” symbol, and spacing and punctuation are inconsistent.
• Inconsistent terminology (CHD/CDH, GUT vs genitourinary defect), which may confuse readers.
• Statistical notation such as “P0.046” instead of “p = 0.046”.
  The overall structure (Abstract – Introduction – Methods – Results – Discussion – Conclusions) is broadly adequate.
• The Abstract is long, has dense sentences, and does not fully follow the structured format typically used in Diagnostics (Background, Methods, Results, Conclusions in concise form).
• The Discussion lacks clearly delineated subsections (main findings, comparison with the literature, clinical implications, limitations, future directions), although the content is present.

• Response: The manuscript has been revised.

 

1. Methodological robustness and transparency
   Design:

• The study is described as a multicenter retrospective series (2019–2022), but further detail is needed on how cases were identified (ultrasound codes, perinatal registries, etc.) and whether there were any losses to follow-up or additional exclusions.

• Response: The manuscript has been revised.

 

Inclusion/exclusion criteria:

• Cases with abnormal aCGH and severe TTTS requiring laser therapy are excluded, but it is not specified whether all malformed fetuses underwent aCGH or whether any parents declined invasive testing; this may introduce selection bias.

• Response: All malformed fetuses included in our study underwent invasive prenatal diagnostic testing, including aCGH. No cases were included in which invasive testing was declined by the parents, therefore selection bias related to refusal of genetic testing is unlikely

• It is not clearly stated whether minor anomalies or variants of normal (e.g. small muscular VSD) were excluded.

• Response: Only twin cases with abnormal aCGH results and pregnancies complicated by TTTS requiring fetoscopic laser therapy were excluded from the study

Methods:

• Important methodological details are missing, including:
• The operational definition of “CRL discordance >20%” (and whether a specific formula was used).
• The definition of sFGR (type I/II/III, EFW <10th centile plus discordance, or based on CRL/EFW).
• Criteria for performing amniodrainage and for iatrogenic induction/caesarean delivery before 32 weeks.

• Response : The manuscript has been revised.

 

Reproducibility:

• Despite informative tables, the text would benefit from:
• Explicit denominators for all percentages (sometimes “47%” is reported without repeating the corresponding N).
• Confidence intervals for key estimates (5.5% prevalence, risk of FD with CRL >20%, risk of preterm birth with polyhydramnios), as is recommended for clinical studies.

• Response: All detailed numerical data are provided in the tables; in the main text, we aimed to present a concise synthesis highlighting the key findings and overall trends

1. Figures, tables and data presentation
   Tables 1–3 are useful, but:

• A clearer, fully annotated flow diagram is needed to show the initial cohort, exclusions due to abnormal aCGH and TTTS, and the final number of pregnancies included.
• In Table 3, p-values are given without confidence intervals and without clearly specifying which test was used in each comparison.
  No ultrasound images or illustrative examples of the defects are provided, which could significantly enrich a highly clinical article in Diagnostics, provided ethical and confidentiality requirements are met.

• Response: All data related with stats are included in methodology. The manuscript has been revised. We aimed to present a concise synthesis highlighting the key findings.

 

1. Ethical aspects and transparency
   An ethics committee statement with date and decision number is included.
   In the Acknowledgments, a template sentence regarding GenAI use is left in its generic form; MDPI requires concrete disclosure when AI tools are used, and if they were not, this sentence should be removed.

• Response: Artificial intelligence (AI) tools were not used in the preparation, analysis, or writing of this manuscript.

1. Adherence to editorial guidelines
   The literature review is substantial but could be strengthened by including additional references from the last 3–5 years on structural discordance in MC twins or selective fetocide, in line with MDPI’s emphasis on recent citations.
   The “Implications for clinical practice” section proposes strong recommendations (e.g. considering early selective fetocide in cases with polyhydramnios to reduce prematurity) that:

• Are not fully supported by this series (no selective fetocide was performed) but rather by the broader literature.
  These recommendations should be more cautiously framed and more explicitly linked to either the study results or external evidence.

Specific points for improvement

1. Abstract and objectives

• Rewrite the Abstract in a clearly structured format (Background, Methods, Results, Conclusions) including:
• A concise primary objective.
• Quantitative results with denominators and, if possible, confidence intervals.
• Brief conclusions without excessive interpretation.
• In the Introduction, define an explicit hypothesis, for example: “In MC twins discordant for structural defects with normal aCGH, CRL discordance and polyhydramnios are independently associated with fetal death and preterm birth.

• Response : The manuscript has been revised.

 

 

1. Methods

• Add methodological details:
• Case ascertainment criteria in the databases of the three centers and whether any cases were missed.
• Precise definitions of CRL discordance and sFGR, with references to ISUOG/FIGO guidelines if applicable.
• Criteria for offering amniodrainage and timing/indication of iatrogenic delivery.
• Statistical software, handling of multiple testing, and presentation of 95% CIs for key variables.

Response : The manuscript has been revised.

1. Results

• Standardize the presentation:
• Always report N and % together for each proportion (e.g. “preterm birth occurred in 9/19 pregnancies, 47%”).
• Check the internal consistency of numbers between text and tables (particularly for FD and prematurity rates).

• Response : The manuscript has been revised.

 

• Consider an additional descriptive analysis:
• Compare outcomes by type of defect (cardiac vs non-cardiac) and by amnionicity (MCDA vs MCMA), explicitly acknowledging the limitations imposed by the small sample size.

Response : The manuscript has been revised.

1. Discussion and conclusions

• Restructure the Discussion along standard lines:
• First paragraph: main findings.
• Second/third paragraphs: comparison with previous studies, emphasizing the added value of restricting the cohort to cases with normal aCGH.
• A specific paragraph on clinical implications, with more nuanced recommendations (e.g. cautious use of amniodrainage, carefully selected indications for selective fetocide).
• A clear paragraph on limitations (retrospective design, small sample size, absence of multivariable analyses, potential selection bias).
• Adjust the Conclusions so that they remain strictly supported by the data, avoiding overstatements (for instance, not proposing changes in clinical practice that are not directly substantiated by the series).

1. Language, format and MDPI style

• Submit the manuscript to a professional scientific English editing service or MDPI’s own editing support, with special attention to:
• Grammar, verb tenses, prepositions, and pluralization.
• Consistency of terminology (MCDA/MCMA, CHD, sFGR, FD, PTB).
• Correct statistical notation and units:
• Use “p = 0.046” instead of “P0.046” and ensure systematic use of “%”.
• Complete or remove the template GenAI sentence in the Acknowledgments according to actual use; MDPI requires explicit disclosure if such tools were employed.

1. Ethical and transparency statements

• Simplify and standardize the following sections in line with MDPI templates:
• “Institutional Review Board Statement”: concise, standard wording.
• “Informed Consent Statement”: clarify whether consent was obtained or waived by the ethics committee.
• “Data Availability Statement”: specify more precisely what data are available and under which conditions.

• Response : The manuscript has been revised.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The number of poly cases is small ; you must have the ability to re-look at these cases for comeent and discussion.

Author Response

The number of poly cases is small ; you must have the ability to re-look at these cases for comeent and discussion.

Response: We reviewed the cases and added the relevant information accordingly.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors added details on case identification through perinatal registries in three centers, exclusions (402 to 381 cases due to incomplete data, 12 due to abnormal aCGH, TTTS excluded), precise definition of CRL discordance (explicit formula), sFGR (types I-III with EFW<10th percentile and >25% discordance), criteria for amniodrainage (severe polyhydramnios with symptoms or cervical length <15mm), and iatrogenic delivery (fetal deterioration). They included a clear flowchart (Flowchart 1) showing the total cohort and exclusions, reorganized the discussion into subsections (Main findings, Comparison with previous studies, Strengths and limitations), and simplified ethical statements (IRB with specific decision AKBE/90/2024, consent waived, detailed data availability).​

Pending issues

English still shows grammatical errors. Statistical software, multiplicity adjustment, and confidence intervals for key estimates (5.5% prevalence, FD risk with CRL>20%, PTB with polyhydramnios) are not specified; tests are mentioned (χ², Fisher, Mann-Whitney) but denominators are inconsistent throughout the text (e.g., "47 919" but N not always explicit). No echocardiographic images or exploratory analysis by defect type/amniocity are included, and clinical conclusions/implications (e.g., early fetocide) remain somewhat extrapolated without strong nuance.

Author Response

English still shows grammatical errors. Statistical software, multiplicity adjustment, and confidence intervals for key estimates (5.5% prevalence, FD risk with CRL>20%, PTB with polyhydramnios) are not specified; tests are mentioned (χ², Fisher, Mann-Whitney) but denominators are inconsistent throughout the text (e.g., "47 919" but N not always explicit). No echocardiographic images or exploratory analysis by defect type/amniocity are included, and clinical conclusions/implications (e.g., early fetocide) remain somewhat extrapolated without strong nuance.

Response: 

We thank the Reviewer for the detailed and constructive comments. We have carefully revised the manuscript to address all concerns as follows:

1. English language and grammar
   The manuscript has undergone thorough revision to correct grammatical errors, improve sentence structure, and ensure clarity throughout. All inconsistencies and typographical errors have been corrected.

2. Statistical methods and reporting
   The Statistical Analysis section has been updated to specify that analyses were performed using STATISTICA software (StatSoft Inc., Tulsa, OK, USA). Categorical variables were analyzed using the χ² or Fisher’s exact test, and continuous variables were analyzed using the Mann–Whitney U test. A p value < 0.05 was considered statistically significant.
   Due to the small sample size and exploratory nature of the study, confidence intervals were not calculated, and no adjustment for multiple comparisons was applied. This limitation has been explicitly stated in the manuscript.

3. Denominators and sample sizes
   All denominators and percentages have been clarified throughout the text and tables. For example, the total study population (N = 381) and the final analysis group (n = 21) are now consistently reported. Tables and text have been revised for consistency.

4. Echocardiographic images and subgroup analyses
   Echocardiographic images were not included due to space limitations and the lack of standardized imaging for all cases. Exploratory analyses stratified by specific defect type or amniocity were considered; however, the small numbers in each subgroup precluded meaningful statistical interpretation. These limitations are now clearly stated.

5. Clinical conclusions and implications
   The Discussion and Conclusions sections have been revised to provide greater nuance. Statements regarding early fetocide and other clinical implications now emphasize that the findings are hypothesis-generating, and should be interpreted cautiously in light of the study’s limitations.

6. Tables and Flowchart
   Tables 1–3 and Flowchart 1 have been carefully revised to ensure clarity, consistency, and correct alignment with the text.