Determinants of Early Child Development

Abstract

Background

Early childhood development is shaped by interactions between biological, prenatal, nutritional, and social factors. Evidence regarding their domain-specific associations across ASQ-3 domains remains limited in Eastern Europe.

Objectives

To examine biological, prenatal, nutritional, and social predictors of developmental outcomes measured with the ASQ-3.

Methods

This cross-sectional study included 167 full-term Bulgarian children aged 14–42 months. Parents completed age-appropriate ASQ-3 questionnaires and a structured survey on prenatal and family characteristics. Raw ASQ-3 scores were converted into age-standardized Z-scores. Stepwise multiple linear regression analyses were conducted for each domain.

Results

Longer breastfeeding duration was positively associated with communication, fine motor, and problem-solving outcomes. Prenatal tobacco exposure was associated with lower communication, fine motor, and problem-solving scores. Communication outcomes were additionally associated with residential setting and bilingual home environment. No significant predictors were identified for gross motor development.

Conclusions

Early developmental domains demonstrate distinct associations with prenatal, nutritional, and social factors.

Keywords

early childhood development ASQ-3 prenatal exposure breastfeeding fine motor development communication skills

Introduction

Early childhood, particularly the period up to 3–4 years of age, represents a highly sensitive developmental window during which core cognitive, motor, language, and socio-emotional competencies are formed. Global estimates indicate that a substantial proportion of children experience developmental vulnerability, with the greatest burden observed in low- and middle-income countries. Systematic review evidence indicates that developmental delay remains highly prevalent among children in low- and middle-income countries.¹ In certain regions, adverse cognitive or socio-emotional outcomes affect more than 40% of preschool-aged children.^2,3 Evidence from large-scale reviews underscores the enduring impact of socioeconomic disadvantage, parental education, and the quality of the family environment on early developmental trajectories and on later educational and health outcomes.^1,3,4

Among potentially modifiable influences in early life, breastfeeding and prenatal exposure to tobacco smoke have been repeatedly linked to neurodevelopmental outcomes. A large national cohort study from Israel demonstrated that longer duration and exclusivity of breastfeeding were associated with a lower likelihood of delayed language, motor, and social milestones at 2–3 years of age, even after extensive adjustment for sociodemographic factors.⁵ Findings from additional cohort studies indicate that breastfeeding is associated with higher cognitive performance in preschool years, independent of maternal intelligence and educational attainment.^6,7 Consistent with this evidence, professional guidelines emphasize breastfeeding as a key contributor to optimal neurodevelopment.⁸

Conversely, prenatal and early postnatal exposure to tobacco smoke has been consistently associated with less favorable cognitive, motor, and language outcomes. Studies in preterm populations have identified prenatal tobacco exposure as an independent predictor of poorer mental, speech, and motor performance at two years of age.⁹ Systematic reviews further confirm its negative effects on early communication, including impairments in receptive and expressive language development.^10,11 Importantly, developmental risks have been reported across different routes of nicotine exposure, including e-cigarettes, with evidence suggesting heightened vulnerability among male children.^11-13

Beyond biological and prenatal factors, social determinants such as maternal education, family structure, and the home language environment play a critical role in shaping early development. Meta-analytic data indicate that low maternal education is associated with nearly a threefold increase in the risk of developmental delay, while adverse socioeconomic conditions during the first 1000 days of life exert particularly strong effects.^1,3 Maternal education has also been shown to influence the accuracy of parent-reported developmental screening instruments, including the ASQ-3, with lower educational attainment linked to a higher risk of misclassification.¹

The Ages and Stages Questionnaires, Third Edition (ASQ-3), is among the most widely used parent-report screening tools for early development worldwide. It assesses five domains—communication, gross motor, fine motor, problem solving, and personal–social development—and is validated for children aged 1 to 66 months.¹⁴ The ASQ-3 is integrated into population-level screening programs, such as the routine developmental assessment conducted at 2–2.5 years of age in England,^15,16 and has been applied across diverse cultural and socioeconomic contexts, including Nepal.² Systematic reviews identify the ASQ-3 as a practical, reliable, and feasible instrument, particularly in settings with limited resources.^4,17,18

Despite this extensive international literature, evidence from Eastern Europe—and Bulgaria in particular—remains scarce regarding the combined influence of biological factors (child sex), prenatal exposures (maternal smoking), nutritional determinants (breastfeeding), and social characteristics (home language environment, family structure, maternal education) on individual ASQ-3 developmental domains. Addressing this gap, the present study examines predictors of developmental outcomes among Bulgarian children aged 14–42 months across all five ASQ-3 domains. Using stepwise multiple regression analyses, the study aims to identify domain-specific patterns of association in early childhood development.

Materials and Methods

Study Design, Recruitment, and Participants

This study employed an observational, analytical, cross-sectional design to examine biological, prenatal, and social determinants of early childhood development across the five ASQ-3 developmental domains. Participant recruitment was carried out between January and October 2025 in three publicly funded daycare centers located in Bulgaria. Parents or legal guardians of all children aged 14–42 months enrolled in these institutions were informed about the study and invited to participate. Developmental assessments were conducted only after written informed consent was obtained. All study procedures were conducted at a university-affiliated medical center, in accordance with institutional regulations and established ethical standards for research involving children.

Eligibility criteria included child age between 14 and 42 months and provision of signed parental consent. Children were excluded if they were born preterm, had congenital anomalies, diagnosed genetic syndromes, or chronic medical conditions with potential impact on neurodevelopment.

Among 246 age-eligible children, 79 were excluded due to absence of parental consent (n = 70), preterm birth (n = 7), chromosomal aberration (n = 1), or congenital heart defect (n = 1). The final analytical sample therefore consisted of 167 full-term children. The ASQ-3 was administered using age-appropriate questionnaires in accordance with standardized administration procedures. The distribution of participants across ASQ-3 age intervals is presented in Table 1.

Table 1.

Characteristics of the Study Sample (N = 167)

Characteristic	Value
Child sex
Male	48% (n = 80)
Female	52% (n = 87)
Age (ASQ-3 questionnaire interval, months)
14 months	1.2% (n = 2)
18 months	2.4% (n = 4)
20 months	2.4% (n = 4)
22 months	6.0% (n = 10)
24 months	4.2% (n = 7)
27 months	17.4% (n = 29)
30 months	9.0% (n = 15)
33 months	13.8% (n = 23)
36 months	20.4% (n = 34)
42 months	23.4% (n = 39)
Рlace of residence
Urban	89% (n = 149)
Rural	11% (n = 18)
Family structure
Married parents	58% (n = 97)
Cohabiting partners	41% (n = 68)
Single parent	1% (n = 2)
Home language environment
Monolingual (Bulgarian)	78% (n = 130)
Bilingual	22% (n = 37)
Bulgarian–Turkish	20% (n = 34)
Other language	2% (n = 3)
Maternal education
Primary	2% (n = 3)
Secondary	48% (n = 81)
Higher	50% (n = 83)
Maternal tobacco smoking during pregnancy
Yes	12% (n = 20)
No	88% (n = 147)
Breastfeeding practices
Not breastfed	17% (n = 28)
Breastfed ≤ 6 months	7% (n = 12)
Breastfed 7–12 months	11% (n = 18)
Breastfed ≥ 12 months	65% (n = 109)
Breastfeeding duration (months)	Mean 6.0 ± 5.0

Values are presented as n (%) unless otherwise indicated. Prenatal tobacco exposure refers to maternal smoking during pregnancy. Breastfeeding duration is reported in months.

Measures

Developmental Assessment

Child development was assessed using the Ages and Stages Questionnaires, Third Edition (ASQ-3), a standardized parent-report screening instrument validated for children aged 1–66 months. The ASQ-3 evaluates five developmental domains: communication, gross motor, fine motor, problem solving, and personal–social development. Each child was assessed with the questionnaire corresponding to their chronological age within the 14–42 month range.

Score Standardization

Raw scores for each ASQ-3 domain were converted into age-standardized Z-scores to allow comparison across different age groups. Age-specific normative means and standard deviations from the ASQ-3 User’s Guide¹⁹ were applied separately for each domain and age-specific questionnaire using the following formula:

Z = \frac{R a w s c o r e - a g e - s p e c i f i c n o r m a t i v e m e a n a g e}{s p e c i f i c n o r m a t i v e s t a n d a r d d e v i a t i o n}

The resulting Z-scores have a mean of 0 and a standard deviation of 1, with positive values indicating performance above and negative values indicating performance below age-specific normative expectations. Because outcomes were standardized for age, chronological age was not included as a covariate in subsequent analyses.

Predictor variables included child sex, place of residence (urban vs. rural), home language environment (monolingual vs. bilingual), family structure (married, cohabiting, single parent), maternal education level, prenatal exposure to tobacco smoke, and duration of breastfeeding.

Statistical Analysis

Statistical analyses were performed using IBM SPSS Statistics, version 19. Statistical significance was defined as p < 0.05. Initial analyses included descriptive statistics and Pearson correlation coefficients to examine associations among age-standardized ASQ-3 domain scores (Table 2).

Table 2.

Pearson Correlations Among Age-Standardized ASQ-3 Developmental Domain Z-Scores.

	Communi-cation	Gross motor	Fine motor	Problem solving	Personal–Social
Communication	1	.332**	.408**	.363**	.391**
Gross Motor	.332**	1	.405**	.170*	.238**
Fine Motor	.408**	.405**	1	.484**	.464**
Problem Solving	.363**	.170*	.484**	1	.194*
Personal–Social	.391**	.238**	.464**	.194*	1

ASQ-3 domains include Communication, Gross Motor, Fine Motor, Problem Solving, and Personal–Social development.

*p < 0.01; **p < 0.05.

Stepwise multiple regression was used as an exploratory modeling approach to identify independent predictors across multiple correlated biological and social variables while limiting overparameterization relative to sample size.

Separate stepwise multiple linear regression analyses were conducted for each ASQ-3 developmental domain. Variables were entered into the regression models using probability-of-F-to-enter and probability-of-F-to-remove criteria of p < 0.05 and p > 0.10, respectively. Prior to model interpretation, assumptions of linear regression were evaluated. Normality of residuals was assessed through visual inspection of residual plots and distribution characteristics. Homoscedasticity was examined using standardized residual scatterplots. Multicollinearity was evaluated using tolerance statistics, with no evidence of problematic collinearity observed among predictor variables. Although the stepwise procedure generated a sequence of models (Models 1–7), interpretation focused on the final model for each developmental domain, which demonstrated the highest explanatory value. Intermediate model steps were examined for consistency and theoretical interpretability. Maternal education was retained in the final personal–social model as a theoretically relevant control variable despite not reaching statistical significance.

To assess the robustness of the findings, sensitivity analyses were additionally performed using logistic regression models with dichotomized ASQ-3 outcomes defined by the referral cutoff of at least 2 standard deviations below the age-specific normative mean. Results of the sensitivity analyses were generally consistent with the primary linear regression models.

Ethical Approval and Informed Consent

All study procedures were conducted in accordance with the principles of the Declaration of Helsinki. The study protocol was reviewed and approved by a local institutional ethics committee (approval № A-471/01 Aug 2025). Parents and legal guardians were provided with detailed written information describing the study aims, procedures, and data protection measures. Participation was entirely voluntary, and families were informed of their right to withdraw from participation at any time without any consequences. Written informed consent was obtained from the parents or legal guardians of all participating children (aged 14–42 months) prior to enrollment in the study.

Results

Moderate, statistically significant positive correlations were observed among the five ASQ-3 developmental domains, indicating shared developmental variance alongside clear domain specificity (Table 2). For each domain, the final stepwise regression solution (Model 7) was retained for interpretation. Overall model fit, assessed using ANOVA, was adequate for all domains with the exception of gross motor development (Tables 3-5).

Table 3.

Stepwise Multiple Linear Regression Model Summary for ASQ-3 Developmental Domain Z-Scores

Domain	R	R²	Adjusted R²	Std. Error	F (df)	p
Communication	0.467	0.218	0.183	0.82	6.26 (7,157)	<0.001
Gross Motor	0.185	0.034	−0.009	0.75	0.80 (7,157)	0.593
Fine Motor	0.339	0.115	0.076	0.90	2.92 (7,157)	0.007
Problem Solving	0.353	0.125	0.086	0.94	3.19 (7,157)	0.003
Personal–Social	0.310	0.096	0.056	0.87	2.39 (7,157)	0.024

Note. Values correspond to final stepwise regression models (Model 7). Age-standardized ASQ-3 Z-scores were used as dependent variables. F statistics reflect overall model fit.

Table 4.

ANOVA Results for Final Stepwise Regression Models Predicting ASQ-3 Domain Z-Scores.

Domain	Regression SS	Residual SS	Total SS	df (Reg, Res)	F	p
Communication	19.72	70.85	90.57	(7,157)	6.26	<0.001
Gross Motor	3.10	87.47	90.57	(7,157)	0.80	0.593
Fine Motor	16.57	127.51	144.08	(7,157)	2.92	0.007
Problem Solving	19.73	138.54	158.26	(7,157)	3.19	0.003
Personal–Social	12.65	118.75	131.39	(7,157)	2.39	0.024

F statistics and corresponding significance levels are reported for each final model.

Note. ANOVA statistics correspond to the final stepwise regression models (Model 7) for each ASQ-3 developmental domain. Age-standardized Z-scores were used as dependent variables.

Table 5.

Final Stepwise Multiple Linear Regression Models Predicting ASQ-3 Developmental Domain Z-Scores

Domain	Predictor	B	SE	β	t	p
Communication	Urban residence	0.681	0.260	0.231	2.62	0.009
	Bilingual home language	−0.637	0.212	−0.272	−3.00	0.003
	Prenatal tobacco exposure	−0.571	0.150	−0.274	−3.80	<0.001
	Breastfeeding duration	0.038	0.013	0.214	2.97	0.003
Gross Motor	No predictors retained	—	—	—	—	—
Fine Motor	Prenatal tobacco exposure	−0.395	0.166	−0.183	−2.39	0.018
Fine Motor	Breastfeeding duration	0.031	0.014	0.172	2.24	0.027
Problem Solving	Prenatal tobacco exposure	−0.470	0.173	−0.207	−2.72	0.007
Problem Solving	Breastfeeding duration	0.036	0.015	0.191	2.50	0.013
Personal–Social	Maternal education	0.188	0.135	0.115	1.39	0.166
Personal–Social	Bilingual home language	−0.446	0.226	−0.193	−1.98	0.050

Unstandardized coefficients (B), standard errors (SE), standardized coefficients (β), t values, and significance levels (p) are shown. Tolerance values indicated no evidence of multicollinearity in any model.

Note. Values are from final stepwise multiple linear regression models (Model 7). Dependent variables are age-standardized ASQ-3 Z-scores. β denotes standardized regression coefficients.

Communication

The final regression model for communication outcomes accounted for a substantial proportion of variance (R² = 0.218; adjusted R² = 0.183) and demonstrated a statistically significant overall fit (F(7,157) = 6.26, p < 0.001; Tables 3 and 4). Residence in an urban setting was independently associated with higher communication Z-scores (β = 0.231, p = 0.009). In contrast, exposure to a bilingual home language environment (β = −0.272, p = 0.003) and prenatal tobacco exposure (β = −0.274, p < 0.001) were associated with lower communication performance. Longer breastfeeding duration was positively related to communication outcomes (β = 0.214, p = 0.003). Child sex, maternal education, and family structure did not show independent associations in the fully adjusted model.

Gross Motor Development

For gross motor development, the final model explained only a negligible proportion of variance (R² = 0.034; adjusted R² = −0.009) and did not reach statistical significance (F(7,157) = 0.80, p = 0.593; Tables 3 and 4). None of the examined biological, prenatal, or social predictors were significantly associated with gross motor Z-scores in the final model (Table 5).

Fine Motor Development

The final regression model for fine motor development explained 11.5% of the variance (R² = 0.115; adjusted R² = 0.076) and showed a statistically significant overall fit (F(7,157) = 2.92, p = 0.007). Prenatal exposure to tobacco smoke was independently associated with lower fine motor Z-scores (β = −0.183, p = 0.018), whereas longer breastfeeding duration was associated with higher fine motor performance (β = 0.172, p = 0.027). Other biological and social predictors did not remain significant after full adjustment.

Problem Solving

The final model predicting Problem Solving outcomes accounted for 12.5% of the variance (R² = 0.125; adjusted R² = 0.086) and demonstrated a significant overall fit (F(7,157) = 3.19, p = 0.003). Prenatal tobacco exposure was negatively associated with Problem Solving scores (β = −0.207, p = 0.007), while breastfeeding duration showed a positive independent association (β = 0.191, p = 0.013). No additional predictors retained statistical significance in the fully adjusted model.

Personal–Social Development

Personal–social development was most strongly related to social determinants. Maternal education emerged as a positive predictor across successive model steps. Although maternal education did not remain statistically significant in the final model, it was retained because of its conceptual relevance. In the final model, exposure to a bilingual home language environment was marginally associated with lower personal–social Z-scores (β = −0.193, p = 0.050). Prenatal tobacco exposure contributed significantly in intermediate models; however, this association was attenuated after the inclusion of breastfeeding duration, suggesting partial overlap between prenatal and early postnatal influences. The final model explained approximately 9.6% of the variance, reflecting the multifactorial and socially embedded nature of personal–social development.

Discussion

The present study identifies distinct, domain-specific patterns in the associations between biological, prenatal, nutritional, and social factors and early childhood development in a community sample of Bulgarian children. While several developmental domains demonstrated sensitivity to modifiable early-life exposures, no statistically significant associations were identified between the examined determinants and gross motor development in this sample.

Although the observed effect sizes were modest, the communication model explained approximately 22% of the variance in developmental scores, suggesting clinically relevant associations between early-life exposures and developmental screening outcomes. The remaining unexplained variance likely reflects the multifactorial nature of early childhood development and the influence of additional environmental, genetic, and relational factors not captured in the present study.

Communication outcomes were most strongly shaped by a combination of prenatal, nutritional, and contextual influences. Longer breastfeeding duration and urban residence were associated with more favorable communication scores, whereas prenatal tobacco exposure and bilingual home language environments were linked to lower performance. The adverse association with prenatal tobacco exposure is consistent with previous research demonstrating negative effects of maternal smoking on early language development¹⁰ and broader neurocognitive functioning.⁹ Similar developmental risks have been reported across different nicotine delivery methods, including e-cigarettes,¹² whereas evidence regarding passive exposure remains less consistent.¹¹ Biological mechanisms proposed in the literature include disrupted neuronal differentiation and synaptic development due to nicotine, as well as reduced fetal oxygenation resulting from carbon monoxide exposure.^20-22

In contrast, breastfeeding was associated with more favorable communication outcomes. This finding aligns with large cohort studies reporting reduced risk of language delay among breastfed children⁵ and broader cognitive advantages that persist beyond early childhood.^6-8,23,24 These associations may reflect both nutritional pathways, including neuroactive components of human milk,²⁵ and relational mechanisms related to close caregiver–infant interaction. Lower communication scores observed among children exposed to bilingual home environments likely reflect distributed language exposure and limitations of screening tools that prioritize expressive abilities in a single dominant language,^26,27 rather than true developmental deficits. The observed advantage associated with urban residence is consistent with evidence highlighting disparities in linguistic input, social interaction, and access to educational resources across residential settings.^27-30

Gross motor development did not demonstrate significant associations with any of the examined predictors, suggesting that basic motor milestones in typically developing children may be primarily driven by maturational processes. Reduced variability in gross motor performance within this community sample, potential ceiling effects of age-appropriate motor milestones, and earlier stabilization of basic gross motor abilities in typically developing children may additionally have contributed to the absence of statistically significant associations. Nevertheless, routine developmental monitoring remains important for the early identification of atypical motor trajectories and neuromotor vulnerability.³¹

Fine motor development was associated with prenatal tobacco exposure and breastfeeding duration. These findings are in line with experimental and epidemiological evidence linking prenatal nicotine exposure to altered motor cortex development,⁹ as well as studies reporting positive associations between breastfeeding and visual–motor coordination and manual precision.⁷ Given established associations between fine motor skills and later academic readiness, executive functioning, and cognitive performance,^32-35 these results emphasize the importance of early preventive efforts.

Problem-solving abilities displayed a comparable pattern, with breastfeeding associated with higher scores and prenatal tobacco exposure associated with poorer outcomes. This domain captures early cognitive processes that are predictive of later attention regulation and executive functioning² and are closely related to early cognitive stimulation and caregiver engagement.³⁶ Findings from international³⁷ ASQ-3 studies similarly indicate heightened vulnerability of cognitive domains to combined biological and environmental risk factors.^29,30

Personal–social development was most closely linked to social context, particularly maternal education and the home language environment. Although child sex was not an independent predictor in the fully adjusted model, higher scores among girls are consistent with evidence of early sex differences in social and communicative behaviors.²⁶ Language-rich and responsive caregiving environments have been repeatedly shown to support early social competence.³⁸ Breastfeeding may additionally be associated with personal–social development through neurobiological pathways and early caregiver–infant interaction.²⁵

Figure 1 synthesizes the domain-specific findings by illustrating how shared and distinct determinants are associated with early developmental functioning. Modifiable prenatal and postnatal exposures—most notably breastfeeding duration and prenatal tobacco exposure—demonstrate consistent associations across communication, fine motor, problem-solving, and personal–social domains, potentially reflecting common biological and relational pathways. In contrast, social context variables such as maternal education and home language environment show more domain-specific effects, underscoring the contextual embedding of early socio-emotional development. Together, this pattern supports a transactional model of early development, in which biological vulnerability and caregiving environments interact to shape cross-domain developmental trajectories.

Figure 1.

Conceptual model illustrating domain-specific associations between modifiable prenatal and social determinants and ASQ-3 developmental domains

Strengths

The present study has several strengths. To our knowledge, this is among the first studies in Bulgaria to examine multiple ASQ-3 developmental domains simultaneously in relation to biological, prenatal, nutritional, and social factors in early childhood. The use of age-standardized Z-scores allowed comparison across different ASQ-3 age intervals and facilitated domain-specific interpretation of developmental variability. In addition, the simultaneous evaluation of multiple early-life determinants enabled a broader examination of developmental patterns across communication, motor, cognitive, and personal–social domains.

Limitations

Several limitations should be considered when interpreting the findings of the present study. First, the cross-sectional design does not permit conclusions regarding temporal or causal relationships between the examined factors and developmental outcomes. Second, both developmental outcomes and exposure variables were based on parent-reported information, which may introduce recall and reporting bias. Third, the ASQ-3 is a developmental screening instrument rather than a diagnostic assessment tool; therefore, the findings should be interpreted as reflecting developmental screening performance rather than confirmed developmental disorders. In addition, participant recruitment through daycare centers and the predominantly urban composition of the sample may limit generalizability to other populations. Finally, subgroup categories such as children exposed to prenatal tobacco smoke and those from bilingual households were relatively small, which may have reduced statistical power for some comparisons. Lower communication-related scores observed in bilingual environments should also be interpreted cautiously, as bilingual language development may not be fully captured by monolingual screening norms.

Clinical and Public Health Implications

Across multiple developmental domains, breastfeeding duration and prenatal tobacco exposure emerged as the factors most consistently associated with developmental outcomes. These findings may help inform early preventive strategies. Developmental screening tools such as the ASQ-3 can support early identification of developmental vulnerability while also serving as a platform for family-centered guidance focused on smoking cessation during pregnancy, breastfeeding support, and promotion of responsive, language-rich caregiving practices, particularly in socially vulnerable populations.

Conclusion

This study provides new evidence on the domain-specific determinants of early childhood development in a Bulgarian community sample. Communication, fine motor, problem-solving, and personal–social development were associated with modifiable prenatal and postnatal factors, including breastfeeding duration and prenatal tobacco exposure, whereas gross motor development showed no significant associations. The findings may inform integrated early childhood strategies that combine developmental screening with targeted preventive interventions addressing prenatal health behaviors and early caregiving environments aimed at supporting optimal developmental trajectories.

Footnotes

Acknowledgments

The author would like to thank the participating families and daycare institutions for their cooperation. The author also wishes to express sincere gratitude to Momchil Itov for his support with organizational activities and for assistance with logistical coordination.

ORCID iD

Tatyana Itova

Ethical Considerations

The study protocol was approved by the Ethics Committee of University Multidisciplinary Hospital “MEDIKA – Ruse” Ltd., Ruse, Bulgaria (approval No. A-471/01 August 2025).

Consent to Participate

Written informed consent was obtained from the parents or legal guardians of all participating children.

Author Contributions

The author was responsible for the study conception and design, data collection, data analysis and interpretation, drafting of the manuscript, and critical revision of the final version.

Funding

The author disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was conducted under the project “WORKSHOP FOR ADVANCED MEDICAL TECHNOLOGIES”, No. Д-ФВП-03/04.03.2025, and was financed by the European Union – NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, within the framework of the project “Scientific University of Ruse”, Contract No. BG-RRP-2.013-0001.

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.*

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