Abstract
Introduction
The aim of this study was to explore long-term behavioral consequences of early versus delayed cord clamping (CC) in school-aged children. The hypothesis was that early CC would be associated with an increased risk of symptoms associated with attention deficit hyperactivity disorder (ADHD) due to the lower iron stores following early CC compared with delayed CC.
Methods
Exploratory, long-term follow-up study of a randomized controlled trial in Sweden. Full-term, vaginally born, neonates to healthy mothers were originally included and randomized to either early (≤10 s) or delayed (≥180 s) CC. At 10 years of age, scores from the screening questionnaire Swanson, Nolan and Pelham Scale IV (SNAP-IV), that identifies symptoms suggestive of ADHD (but do not diagnose ADHD), were compared between groups of early and delayed CC.
Results
We assessed 139/382 (36.4%) children, 64 early CC and 75 delayed CC. No significance in total score and subgroup scores of SNAP-IV was seen when comparing early and delayed CC. In the primary outcome, total scores, mean (standard deviation) of early CC was 14.64 (13.94) and for delayed CC 13.59 (13.41), p = 0.65.
Conclusion
Timing of umbilical CC and iron status were not associated with symptoms associated with ADHD in 10-year-old children. Studies with higher follow-up rates and in populations with high prevalence of iron deficiency are needed to verify or discard the findings. Understanding if delayed CC has long-lasting neurobehavioral or neurodevelopmental effects can help develop guidelines and program about neonatal care.
Keywords: Delayed cord clamping, Iron deficiency, Neurodevelopment
Introduction
Iron deficiency anemia affects 1.2 billion people and is a major global issue from infancy to adulthood, contributing significantly to “years lived with disability”. Iron deficiency in the late fetal period and early childhood can impact neurodevelopment, cognition, memory, and fine motor skills and lead to increased socioemotional and behavioral problems [1, 2]. Early iron supplementation in low birth weight children (2–2.5 kg) reduces behavioral problems at age 7 [3], and supplementation at 6–12 months in children without iron deficiency anemia improves adaptive behavior at age 10 [4]. Furthermore, a dose-response relationship between iron deficiency and worse socioemotional outcomes has been seen [5]. Optimizing infant iron stores can be achieved by delayed umbilical cord clamping (CC) [6], which allows for placental transfusion and improves infant iron stores [7].
In preterm neonates, a meta-analysis of 48 randomized controlled trials (RCTs) comparing delayed and early CC found that delayed CC reduced death before discharge [8]. There is no single definition for delayed CC; however, most studies define it as ≥60 s and this is now the recommendation of international guidelines [9].
Studies show that delayed CC improves iron stores and white matter brain growth on magnetic resonance imaging at 4 and 12 months in brain regions involved in motor function, visual/spatial, and sensory processing [10, 11]. The increased iron endowment in early infancy following delayed CC [7, 10, 12] is currently the main explanation of the improved neurodevelopment seen up to 4 years of age after delayed CC [13].
Studies on the long-term outcomes of delayed CC would strengthen its rationale, as this remains a knowledge gap [9]. Currently, there are no follow-up data on delayed CC beyond age 4. This study aimed to investigate if delayed CC in healthy neonates in a high-income country was associated with improved behavior in school-aged children. Since low umbilical cord ferritin is associated with poorer attention scores at age 5 [14], we hypothesized that early CC would increase the risk of attention problems, measured as symptoms related to attention deficit hyperactivity disorder (ADHD).
Methods
Study Design
Follow-up of an RCT, the Cord Clamping Study, where children born at full-term had been randomized to early versus delayed CC. The study included children who had been enrolled in both the Cord Clamping Study and the Halland Health and Growth Study. The Halland Health and Growth Study collected the data on ADHD symptoms. Merging of the data from the two studies was approved by the Swedish Ethical Review Authority (dnr 2023-02832-02). The original study was registered with ClinicalTrials.gov, NCT01245296, https://classic.clinicaltrials.gov/ct2/show/NCT01245296. The study has followed the CONSORT checklist (online suppl. material; for all online suppl. material, see https://doi.org/10.1159/000547383).
Original Study
The Cord Clamping Study took place at the Hospital of Halland from April 2008 to May 2009, after ethics approval from the Regional Ethics Review Board at Lund University (protocol 41/2008), written patient consent from parents, and registration at ClinicalTrials.gov (NCT01245296). Full-term neonates with a gestational age of 37–41 weeks were eligible if the mother was healthy, non-smoking and had an uncomplicated pregnancy with expected vaginal delivery. Randomization assignments (1:1) consisted of early (≤10 s after delivery) or delayed (≥180 s after delivery) CC, using computer-generated blocks of 20, and the allocation was given by sequentially labeled opaque and sealed envelopes [12]. Due to the study design, neither mothers nor midwives performing could be blinded, but staff performing neonatal exams, collecting data, and analyzing blood samples were blinded to group allocation [12]. The primary outcome was hemoglobin and iron status at 4 months, measured by serum ferritin, transferrin saturation, and other markers [12]. Secondary outcomes included psychomotor development at 4 [15] and 12 months [16], assessed by the Ages and Stages Questionnaire (ASQ), iron deficiency at 4 and 12 months, defined as ferritin <12 microgram/L (µg/L), or two other abnormal iron indicators [12, 16].
Participants
Study participants included 280 of the 382 children who were randomized in the Cord Clamping Study (2008–2009) [12] as these also participated in the parallel observational Halland Health and Growth Study and thereby received the neuropsychiatric screening questionnaires, shown in Figure 1.
Fig. 1.
Flowchart with participants. At 10 years of age, parents to 139 children responded to the neuropsychiatric screening questionnaire Swanson, Nolan and Pelham Scale IV (SNAP-IV).
Outcomes
The primary outcome was the difference between the two cord clamping groups (early vs. delayed CC) in total score of the ADHD screening questionnaire Swanson, Nolan and Pelham Scale IV (SNAP-IV). The Swedish version of SNAP-IV was used [17]. Secondary outcomes were the proportion of children with SNAP-IV subgroup scores above the American 95th percentile cutoffs for parent reported SNAP-IV [18], presented as the average-per-item with two decimal places in line with the manual. Subgroups of the SNAP-IV relate to the two subset of ADHD symptoms: inattention and hyperactivity/impulsivity [18]. The SNAP-IV scores were analyzed in relation to sex; ferritin levels at birth, 4 and 12 months; as well as ASQ test scores at 4 and 12 months.
Data Collection
Background variables linked to ADHD were selected [19]. The SNAP-IV questionnaire was e-mailed to parents by the Halland Health and Growth Study, with up to three reminders for non-responders. Data were collected from December 2018 to February 2019 for children born 2008 and from December 2019 to February 2020 for children born 2009. The 26 SNAP-IV items covers two ADHD subtypes: inattention (items 1–9) and hyperactivity-impulsivity (11–19 items) [17, 18]. The combined ADHD form is the sum of inattention (items 1–9) and hyperactivity-impulsivity (item 11–19) scores [17, 18]. Items 21–28, associated with oppositional defiant disorder [ODD], were also included due to symptom overlap with ADHD. SNAP-IV uses a 0 to 3 rating scale (0 = not at all, 3 = very much). Subgroup scores are the average item score within each subtype.
Statistical Analysis
The study was a follow-up of an RCT, and the sample size considered fixed. Mean and standard deviation were used for normally distributed variables, and according to the screening questionnaires, median and range were used for skewed and ordinal variables, and numbers and percentages were used for categorical variables. For comparison between groups, Fisher’s exact test was used for categorical variables. For comparison of continuous test scores, mean differences between early and delayed CC groups were calculated with Student’s t-test. For ordinal scale variables, Mann-Whitney U test was used. Exploratory correlations were analyzed by Pearson’s r and Spearman’s rho as appropriate and significant variables were planned to be entered in regression models. p values <0.05 were considered statistically significant. Analyses were performed both per intention to treat and per protocol. As the outcomes were skewed and variables ordinal, data were also analyzed with non-parametric tests and similar results were then achieved. However, data are presented with mean and standard deviation in accordance with the SNAP-IV manual. We used IBM SPSS Statistics for Macintosh, version 29.0 (Armonk, NY, USA: IBM Corp).
Post hoc Analyses
Six cases had missing data on one question each (all different questions), and 1 case had missing data on 2 questions. In total, missing cases were 5.0% and missing values were 0.2%. The SNAP-IV manual does not include recommendations on how to handle missing data. Due to the low frequency of missing values, but also the risk of bias and greater reduction of the already low power that would follow listwise deletion [20], we replaced the missing values with the mean value for that case in the subtype of the SNAP-IV (i.e., inattention, hyperactivity/impulsivity, or ODD) where the question was missing.
Results
In total, 280 of the 382 (73.3%) children in the Cord Clamping trial were eligible to take part in the present investigation. Of these, 139/280 (49.6%) responded to the neuropsychiatric screening questionnaire SNAP-IV, shown in Figure 1, and were included in the present evaluation. The SNAP-IV response rate did not differ between the two CC groups. Of the 139 children, 130 (93.5%) originally received CC according to their randomization group, i.e., per protocol. Results are presented as intention to treat in the article, and per-protocol results can be found in online supplementary 1 (S1). There were no differences in background data between children included in the study and children lost at follow-up (missing cases), online supplementary 2 (S2). There were no significant differences in background data between the groups early and delayed CC. Background characteristics of the participants are presented in Table 1.
Table 1.
Background characteristics of children who participated in a randomized controlled trial comparing early (≤10 s) versus delayed (≥180 s) CC
| | Early cord clamping (N = 64) | n | Delayed cord clamping (N = 75) | n | p value | ||
|---|---|---|---|---|---|---|---|
| Maternal characteristics | |||||||
| Age, years | 32.0 | 3.9 | 64 | 31.4 | 4.6 | 75 | 0.44 |
| Weight, kg | 64.6 | 10.5 | 63 | 66.3 | 11.5 | 75 | 0.37 |
| Body mass index, kg/m2 | 23.3 | 3.1 | 57 | 23.7 | 3.8 | 63 | 0.50 |
| Neonatal characteristics | |||||||
| Gestational age, weeks | 40.0 | 1.1 | 64 | 40.1 | 0.9 | 75 | 0.46 |
| Sex, female, n (%) | 29 | 45.3 | | 40 | 53.3 | | 0.40 |
| Birth weight, kg | 3.6 | 0.51 | 64 | 3.7 | 0.45 | 75 | 0.10 |
| Apgar score 1 mina | 9 | 6 to 10 | | 9 | 4 to 10 | | 0.57 |
| Apgar score 5 mina | 10 | 6 to 10 | | 10 | 7 to 10 | | >0.99 |
| Apgar score 10 mina | 10 | 7 to 10 | | 10 | 8 to 10 | | >0.99 |
| Cord ferritin, µg/L | 233.6 | 198.1 | 63 | 221.6 | 135.1 | 74 | 0.68 |
| 4-month data | |||||||
| Ferritin, µg/L | 120.8 | 112.5 | 62 | 150.3 | 134.4 | 72 | 0.17 |
| ASQ score | | | 63 | | | 73 | |
| Communication | 50.9 | 6.0 | | 50.6 | 7.6 | | 0.83 |
| Gross motor | 53.8 | 8.4 | | 55.5 | 6.5 | | 0.18 |
| Fine motor | 47.2 | 12.3 | | 49.7 | 9.7 | | 0.21 |
| Problem solving | 53.6 | 7.9 | | 54.7 | 7.4 | | 0.38 |
| Personal social | 51.5 | 8.0 | | 49.9 | 10.6 | | 0.33 |
| Total score | 257.0 | 28.7 | | 260.5 | 30.0 | | 0.49 |
| 12-month data | |||||||
| Ferritin, µg/L | 41.1 | 27.2 | 59 | 46.6 | 42.0 | 68 | 0.40 |
| ASQ score | | | 61 | | | 73 | |
| Communication | 40.8 | 11.8 | | 39.5 | 12.9 | | 0.54 |
| Gross motor | 49.2 | 12.9 | | 44.7 | 15.9 | | 0.07 |
| Fine motor | 51.3 | 8.6 | | 53.1 | 7.9 | | 0.22 |
| Problem solving | 46.8 | 13.5 | | 46.3 | 11.2 | | 0.82 |
| Personal social | 42.5 | 11.4 | | 43.8 | 11.9 | | 0.51 |
| Total score | 230.6 | 39.6 | | 227.4 | 42.6 | | 0.65 |
Values are in mean (standard deviation) unless stated otherwise. Results are presented as intention to treat.
ASQ, Ages and Stages Questionnaire, third edition.
aMedian (range).
Primary Outcome
The total scores of SNAP-IV did not differ between children in the two randomization groups: early CC, 14.64 points (13.94 in standard deviation); versus delayed CC, 13.59 (13.41), as presented in Table 2.
Table 2.
Results of neurodevelopmental screening questionnaire SNAP-IV, comparing 10-year-old children who were included in a randomized controlled trial of early (≤10 s) versus delayed (≥180 s) CC
| | Early CC (n = 64) | Delayed CC (n = 75) | Mean diff. (95% CI) | p value | |||
|---|---|---|---|---|---|---|---|
| SNAP-IV | |||||||
| Scores | |||||||
| Total | 14.64 | 13.94 | 13.59 | 13.41 | 1.05 | −3.54 to 5.65 | 0.65 |
| Inattention | 6.23 | 5.83 | 4.97 | 5.47 | 1.26 | −0.63 to 3.17 | 0.19 |
| Hyperact./impulsiv. | 4.19 | 5.80 | 3.83 | 5.59 | 0.36 | −1.55 to 2.28 | 0.71 |
| Combined | 10.42 | 10.70 | 8.79 | 9.91 | 1.62 | −1.83 to 5.09 | 0.36 |
| ODD | 4.23 | 4.01 | 4.80 | 4.73 | −0.57 | −2.05 to 0.92 | 0.45 |
| Average scores | |||||||
| Inattention | 0.69 | 0.65 | 0.55 | 0.61 | 0.14 | −0.07 to 0.36 | 0.19 |
| Hyperact./impulsiv. | 0.47 | 0.64 | 0.43 | 0.62 | 0.04 | −0.17 to 0.26 | 0.71 |
| Combined | 0.58 | 0.59 | 0.49 | 0.55 | 0.09 | −0.10 to 0.29 | 0.36 |
| ODD | 0.53 | 0.50 | 0.60 | 0.59 | −0.07 | −0.25 to 0.12 | 0.45 |
Values are mean (standard deviation) if not stated otherwise. Inattention, hyperactivity/impulsivity (hyperact./impulsiv.), and combined (combination of inattention and hyperactivity/impulsivity) forms of attention deficit hyperactivity disorder. Average rating-per-item is calculated by summing the scores on the items in the subset, then dividing by the number of items in the subset, according to SNAP-IV. Results are presented as intention to treat.
n, number; ODD, oppositional defiant disorder; SNAP-IV, Swanson, Nolan and Pelham Scale IV.
Secondary Outcomes
The sub-scores of SNAP-IV, scores on ODD, and the average scores on SNAP-IV and ODD did not differ between the groups, Table 2. The proportion of children with scores above cutoffs for each of the three subtypes of ADHD and for ODD did not differ between the two CC groups (Table 3). When comparing total scores and sub-scores between the CC groups for iron deficiency, no significant differences were found. None of the background variables, maternal age/weight/body mass index, gestational age, birthweight, ferritin levels in the cord and at 4 and 12 months, was significantly correlated to the outcomes, and consequently no regression analyses were performed. Results were similar when analyzed per protocol (i.e., the 130 children who received their allocated intervention compared to the 139 included in the present study, results not shown). There were no significant differences when comparing early clamped girls with delayed clamped girls and early clamped boys with delayed clamped boys; results above cutoff scores are presented in Table 4, and results of total scores, including scores for each subtype, are presented online supplementary 3 (S3).
Table 3.
Scores above cutoff in the screening questionnaire SNAP-IV, comparing 10-year-old children who were included in a randomized controlled trial of early (≤10 s) versus delayed (≥180 s) CC
| | Early CC (n = 64), n (%) | Delayed CC (n = 75), n (%) | p value |
|---|---|---|---|
| SNAP-IV | |||
| Inattention (mean score ≥1.78) | 5 (8) | 5 (7) | >0.99 |
| Hyperactivity/impulsivity (mean score ≥1.44) | 8 (13) | 7 (9) | 0.59 |
| Combined (mean score ≥1.67) | 5 (8) | 4 (5) | 0.73 |
| ODD (mean score ≥1.88) | 1 (2) | 5 (7) | 0.22 |
Values are in numbers (%). Inattention, hyperactivity/impulsivity, and combined (combination of inattention and hyperactivity/impulsivity) forms of attention deficit hyperactivity disorder. For SNAP-IV, the average rating-per-item is calculated by summing the scores on the items in the subset, then dividing by the number of items in the subset, according to SNAP-IV. Cutoff scores in average rating-per-item are based on the American 95th percentile for parent reported questionnaire.
n, number; ODD, oppositional defiant disorder; SNAP-IV, Swanson, Nolan and Pelham Scale IV.
Table 4.
Scores above cutoff in the screening questionnaire SNAP-IV, comparing 10-year-old girls and boys who were included in a randomized controlled trial of early (≤10 s) versus delayed (≥180 s) CC
| | Girls | Boys | ||||
|---|---|---|---|---|---|---|
| early CC (n = 29) | delayed CC (n = 40) | p value | early CC (n = 35) | delayed CC (n = 35) | p value | |
| n (%) | n (%) | n (%) | n (%) | |||
| SNAP-IV | ||||||
| Inattention (score ≥1.78) | 4 (14) | 1 (3) | 0.15 | 1 (3) | 4 (11) | 0.36 |
| Hyperactivity/impulsivity (score ≥1.44) | 2 (7) | 0 (0) | 0.17 | 6 (17) | 7 (20) | >0.99 |
| Combined (score ≥1.67) | 2 (7) | 0 (0) | 0.17 | 3 (9) | 4 (11) | >0.99 |
| ODD (score ≥1.88) | 1 (3) | 1 (3) | >0.99 | 0 (0) | 4 (11) | 0.11 |
Values are in numbers (%). Inattention, hyperactivity/impulsivity, and combined (combination of inattention and hyperactivity/impulsivity) forms of attention deficit hyperactivity disorder. The average rating-per-item is calculated by summing the scores on the items in the subset, then dividing by the number of items in the subset, according to SNAP-IV. Cutoff scores in average rating-per-item are based on the American 95th percentile for parent reported questionnaire. Results are presented as intention to treat.
n, number; ODD, oppositional defiant disorder; SNAP-IV, Swanson, Nolan and Pelham Scale IV.
Discussion
We compared neurobehavioral outcomes in a cohort of 10-year-old term children from an RCT on early versus delayed CC, hypothesizing that decreased iron stores (in early infancy) from early CC would increase ADHD associated symptoms at age 10. Parents completed the SNAP-IV questionnaire to identify symptoms suggestive of ADHD and ODD. No differences were found between groups in ADHD or ODD scores or sub-scores. The percentage of children with symptoms for different ADHD subtypes was similar or slightly higher than Sweden’s ADHD prevalence (5–7%) [21], indicating our sample was representative as we used a screening tool, not diagnostic criteria. At 1 year, no differences in ASQ scores or sub-scores were seen between groups [16], unlike a Nepalese RCT, which found higher ASQ scores in delayed CC children [22]. Our 4-year follow-up showed delayed CC improved fine motor function, especially in boys, but did not affect behavior scores [13]. The Nepalese RCT’s 3-year follow-up showed no differences in ASQ scores related to timing of CC, though more girls were “at risk” for gross motor delay after early CC [23]. In our study, no relationship was found between iron stores at birth and neurobehavioral results at 10 years, maybe due to low iron deficiency prevalence. In populations with low iron deficiency prevalence, childhood neurodevelopment may not closely relate to iron levels affected by CC timing. There remains a possibility that although delayed CC improves iron stores early in life, the beneficial effect may not persist until 10 years of age. Environmental, dietary, and other factors might dilute the effect of delayed CC over time.
Iron deficiency in early life is associated with risk for suboptimal neurodevelopment [1, 24], affecting processes like myelogenesis, neurogenesis, and differentiation, which depends on sufficient iron. Animal models show iron deficiency alters brain structure and function, impacting noradrenergic, dopaminergic, and serotonergic neurotransmissions [25]. Suboptimal monoaminergic and glutaminergic neurotransmission may explain the association between ADHD and iron deficiency [26]. Given the evidence that early iron deficiency affects long-term neurodevelopment [1–3, 5, 27] and that delayed CC improves early iron stores [6, 7, 10, 12], we hypothesized that the improved early iron stores from delayed CC would reduce attention problems but could not see this in the present study. Other theoretical mechanisms may also link delayed CC and neurodevelopment such as the transfusion of stem cells and placental progesterone, both hypothesized to optimize brain environment [28, 29]. Mercer et al. [29] have further suggested that mechanical blood force created during placental transfusion stimulates organ-specific endothelial cell to growth and repair, supporting organ health and development.
A limitation of this study is the reliance on parental SNAP-IV reports. Furthermore, although validated, SNAP-IV is only an indicator of ADHD and cannot substitute for more detailed neurodevelopmental investigations. Other methods, such as in-person assessments, may have detected group differences; however, we lacked the resources for this. SNAP-IV is a screening tool and ADHD diagnosis requires comprehensive clinical assessment using multiple sources, including adherence to the Diagnostic and Statistical Manual of Mental Disorders, the DSM, multi-informant and multi-method approaches, and direct clinical assessment to rule out other causes. Our hypothesis was not that early CC would lead to increased ADHD rates, but that symptoms associated with ADHD, as a sign of a neurodevelopmental impact, might be more present due to the lower iron stores in early infancy seen after early CC. As our results are, to our knowledge, the only results on long-term development in school-aged children comparing early versus delayed CC, the impact of improved iron stores following different cord management strategies needs to be evaluated further.
There is also a possibility that our hypothesis regarding improved iron stores following delayed CC is not the main explanation to previous findings in improved neurodevelopment after delayed CC. Another limitation was the loss at follow-up despite several reminders to fulfill the questionnaire. Reminding participants by both e-mail and phone calls could have mitigated some of the loss. However, when comparing background data between children in the present study and missing subjects from the Cord Clamping Study, the populations were similar, making the results reliable. Finally, the original Cord Clamping Study was designed to show increased ferritin levels after delayed CC [12], but not neurodevelopmental differences.
Delayed CC is a simple, inexpensive, and accessible intervention, available also in less developed regions. Maintaining sufficient iron throughout fetal and early infant development is considered more effective for early brain development than correcting iron deficiency later [30] – making delayed CC an early, interesting option that needs further long-term evaluation. Potential differences in children subjected to early versus delayed CC may be revealed with more detailed cognitive assessments. This needs to be evaluated further.
Conclusion
Delaying CC for 3 min in low-risk, full-term neonates resulted in similar neurobehavioral development at 10 years as in children randomized to early CC. The children included in this study were born in a high-income country with low prevalence of iron deficiency and many were lost at follow-up. Delayed CC has the potential to improve development with no economic costs for society and no harm to the child, meaning it is easy to adopt even when there is a lack of resources. However, future studies performed in settings with high prevalence of iron deficiency, with less loss at follow-up and that evaluates other aspects of iron deficiency such as cognitive functions rather than neurobehavioral problems, are needed.
Statement of Ethics
This study was reviewed and approved by the Swedish Ethical Review Authority (dnr 2023-02832-02). The merging of the data from the Cord Clamping Study and the Halland Health and Growth Study did not require renewed written consent according to the Swedish Ethical Review Authority (dnr 2023-02832-02).
Conflict of Interest Statement
Dr. Isacson received a grant from Lilla Barnets Fond (“Small Child Foundation”) to support the study (declared under funding). Ola Andersson was a member of the journal’s Editorial Board at the time of submission. The other authors have no conflicts of interest to declare.
Funding Sources
This study was supported by a grant from Lilla Barnets Fond. The funder had no role in the design, data collection, data analysis, and reporting of this study.
Author Contributions
Dr. Isacson conceptualized and designed the study, carried out the initial analyses, drafted the initial manuscript, and reviewed and revised the manuscript. Prof. Hellström-Westas conceptualized and designed the study, supervised the analyses, interpreted the data, and reviewed and revised the manuscript. Prof. Domellöf and Prof. Dahlgren conceptualized the study, interpreted the data, and revised the manuscript. Dr. Roswall coordinated and supervised the data collection, interpreted the data, and reviewed and revised the manuscript. Dr. Andersson conceptualized and designed the study, collected, coordinated and supervised the data collection, interpreted the data, supervised the analyses, and reviewed and revised the manuscript. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.
Funding Statement
This study was supported by a grant from Lilla Barnets Fond. The funder had no role in the design, data collection, data analysis, and reporting of this study.
Data Availability Statement
The data that support the findings of this study are not publicly available due to information that could compromise the privacy of research participants, but the data are available from Ola Andersson upon reasonable request.
Supplementary Material.
Supplementary Material.
Supplementary Material.
Supplementary Material.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The data that support the findings of this study are not publicly available due to information that could compromise the privacy of research participants, but the data are available from Ola Andersson upon reasonable request.
