External Publications Using GUI Data

This nationally representative study of Irish infants explores whether the set of child and environmental factors established as predicting language outcomes aged 3 years would also predict language and communication development as early as age 9 months. Associations between infant and environmental characteristics and infant language outcomes at 9 months, as measured on the ASQ 10-month communication questionnaire, were explored in a series of bivariate logistic regression models and in a fully adjusted multivariable logistic regression model. Infant gender, health, birthweight and temperament were significantly associated with passing the ASQ communication measure at 9 months. Being an only child and having some experience of relative childcare predicted positive communication outcomes. Infants of older mothers and mothers who spoke less to the child while doing other things were significantly less likely to pass. Infants of mothers with the lowest educational level were more likely than their peers to pass at 9 months. Unlike language outcomes at 3 years, low maternal education does not appear to be a risk factor for poorer outcomes at 9 months. This study adds significantly to the infant language development literature by focussing on communication outcomes in early infancy and identifying protective and risk factors at such an early stage.

Introduction
Despite a growing body of research on associations between adolescent digital use and well-being, few studies have investigated these associations a) longitudinally and b) across socioeconomic status. The present study uses high-quality longitudinal data to examine how digital engagement shapes socioemotional and educational outcomes from early to late adolescence across socioeconomic status (SES).

Methods
Participants are 7685 individuals (49.0% female) from the 1998 birth cohort of the longitudinal Growing Up In Ireland (GUI) survey. The survey was administered to Irish parents and children between 2007 and 2016 (at ages 9, 13, and 17/18). Fixed-effects regression modeling was used to establish associations between digital engagement and socioemotional and educational outcomes. Further Fixed-Effects models were analyzed separately by SES, to assess how associations between digital use and adolescent outcomes differ by socioeconomic groups.

Results
Results show that digital screen time increases markedly from early to late adolescence, but to a higher extent among low-SES versus high-SES groups. Heavy levels of digital screen time (i.e., 3+ hours daily) are associated with declines in well-being, particularly for external and prosocial functioning, while engagement in learning-oriented digital activities and gaming is associated with better adolescent outcomes. Yet, low-SES adolescents are globally more harmed than high-SES adolescents by their digital engagement, and high-SES adolescents benefit more from moderate levels of digital use and from engaging in learning-oriented digital activities.

Conclusions
This study suggests that digital engagement is associated with socioeconomic inequalities in adolescents’ socioemotional well-being and, to a lesser extent, educational outcomes.

Background
Preventing child mental health problems and child obesity have been recognized as public health priorities. The aim of the present study was to examine whether being breastfed (at all or exclusively) in infancy was a predictor of mental well-being and protective against risk of obesity at age 9.

Methods
Cross-sectional data from a large, nationally representative cohort study in the Republic of Ireland was used (n = 8357). Data on breastfeeding were retrospectively recalled. Child mental well-being was assessed using a parent-completed Strengths and Difficulties Questionnaire (SDQ). Child’s height and weight were measured using scientifically calibrated instruments.

Results
Logistic regression analyses indicated that, after controlling for a wide range of potential confounding variables, being breastfed in infancy was associated with a 26% (P < 0.05) reduction in the risk of an abnormal SDQ score at 9 years. Being breastfed remained a significant predictor of child mental well-being when child obesity was controlled for, indicating that being breastfed, independent of child obesity, is a predictor of child mental well-being. The results of a second logistic regression indicated that, after controlling for a wide range of potential confounding variables, being breastfed for between 11 and 25 weeks was associated with a 36% (P < 0.05) reduction in the risk of obesity at 9 years, while being breastfed for 26 weeks or longer was associated with a 48% (P < 0.01) reduction in the risk of obesity at 9 years.

Conclusions
Breastfeeding in infancy may protect against both poor mental well-being and obesity in childhood.

Background
Health disparities for children with intellectual disabilities can be challenging to measure due to many other factors that can impact health and healthcare use. The aim of the current study was to use longitudinal cohort data to compare children with intellectual disability (ID) in Ireland between 2006 and 2014 on healthcare utilisation and unmet need, at ages 9 and 13, using a propensity score matching (PSM) approach.

Methods
Using data from the Growing up in Ireland study, PSM was used to identify an appropriate control sample to compare with a sample of children with ID (n = 124). Participants were matched on variables that are known to influence healthcare utilisation to reduce the impact of confounding variables between groups so that differences between the groups can be estimated. Logistic regression was used to estimate effects at ages 9 and 13.

Results
Children with ID were no more likely to have visited a general practitioner or emergency department in the past 12 months than children without ID. They did have a greater likelihood of visiting a doctor in a hospital in the past 12 months and of having an overnight stay in hospital by age 9. Primary caregivers of children with ID were more likely to report unmet health needs at ages 9 and 13.

Conclusions
This approach is a novel means of comparing healthcare use in this population by balancing the impact of other factors that may result in inequities, to which children with ID may be more vulnerable.

This paper, for the first time, presents estimates of the causal impact of overweight and obesity on self-rated health (SRH) using instrumental variables (IV) econometric methods. While a number of previous studies have sought to better understand the determinants of SRH, there is no consensus in relation to the impact of overweight and obesity. Using data from a large nationally representative sample of Irish parents and their children, we estimate a range of ordered probit models to isolate the causal effect of overweight and obesity on SRH. Our data includes independently and objectively recorded weight and height measures for parents and their children and we instrument for parental body mass index (BMI) status using the BMI of a biological child. After controlling for a range of individual, socioeconomic, health and lifestyle related variables, we find that being overweight has a negligible impact on SRH, while being obese has a practically and statistically significant negative impact on SRH, with these effects most pronounced for those who are most obese. We find only minor differences in these effects across gender.

Gender inequality in housework divisions is persistent. This study examined early-emerging patterns in gender roles via children’s gendered housework time in the Irish context using time-use diary and questionnaire data, controlling for key demographic variables. This study utilized longitudinal “Growing up in Ireland” data, comparing the same children’s behaviour at age 9 and 13. Participants were a nationally representative, weighted sample of school going children living in Ireland, including 4,135 girls, 3,831 boys and their caregivers. The results highlight that inequalities in domestic chores occur at nine and are even greater at 13: girls do more housework and children tend to do “gender typical” chores. Consequences are discussed in relation to gender inequality and policy implications.

Large-scale international migration continues apace. From a health-care services perspective, it is important to understand the influence of migrant heritage on utilization, to allocate resources appropriately and facilitate equity. However, the differences in utilization across different migrant groups remain poorly understood, particularly so for paediatric populations. This paper contributes to filling this gap in knowledge, examining the health-care contact of children for whom their primary caregiver is foreign-born, using longitudinal data from two nationally representative surveys. The study setting is Ireland, which provides an interesting case as a small, open European country, which for the first-time experienced net inward migration in the past two decades. For both cohorts, panel regression models, adjusting for socioeconomic and health indicators, demonstrated lower utilization of general practitioner (GP) services for children of caregivers from ‘less-advanced, non-Anglosphere, non-European Union (EU)’ nations, relative to native-born counterparts. Relatively lower attendances at Emergency Departments and hospital nights were also observed for this group, as well as for children born to EU (non-UK) caregivers. The insights provided are instructive for policymakers for which immigration is a substantial phenomenon in current and future population demographics.

Objective
This study aims to examine the associations between prenatal exposure to maternal smoking, birth weight and persistent offspring psychiatric symptoms. Additionally, we aim to examine whether the relationship between prenatal maternal smoking and persistent offspring psychiatric symptoms is mediated by offspring birth weight.

Methods
This study used the Growing Up in Ireland (GUI) longitudinal cohort. The GUI is a nationally representative longitudinal study of children which consisted of three data collection waves, at ages 9, 13, and 17 years. Logistic regression analysis was used to examine associations between prenatal tobacco exposure, and offspring psychiatric symptoms. Linear regression was used to examine associations between prenatal tobacco exposure and offspring birth weight. We conducted a mediation analysis examining potential etiological pathways linking maternal smoking during pregnancy, offspring birth weight, and later offspring psychiatric symptoms. All analyses were adjusted for confounders including household income, maternal level of education, and family psychiatric history. Additionally, examination of birth weight and subsequent psychiatric symptoms also was controlled for prematurity.

Results
We found that the association between prenatal tobacco exposure and later psychiatric symptoms is mediated by birth weight.

Conclusions
This work provides further evidence that maternal smoking during pregnancy is an important modifiable lifestyle factor that has an impact not just on the physical health of offspring but also their mental wellbeing. Supporting women with structured smoking cessation programs at the earliest stages of pregnancy should be a public health priority.

Background
Parental incarceration (PI) is associated with adverse developmental outcomes for children affected. However, research in this area often reports conflicting results with few studies following children across time in non-U.S. populations. Additionally, more research is called for using multi-informant perspectives rather than relying on adult reports of child outcomes alone.

Methods
This study used data from the first two waves of a nationally representative cohort study of 8,568 children aged 9 years and followed up at age 13 living in the Republic of Ireland (2007–2012). Propensity score matching was used to match children who had experienced PI by the age of nine to children who had not experienced PI by sociodemographics and experience of other stressful events. Mental health, as measured by self-concept (Piers-Harris II) and externalizing and internalizing difficulties (strength and difficulties questionnaire), was compared across both groups.

Results
Fifty of the 8,568 children (weighted percentage 0.9%) reported experiencing PI by the age of nine. These children came from more socially disadvantaged homes and were more likely to have experienced other potentially stressful life events. In comparison to a matched sample of children not affected by PI, children affected by PI reported higher levels of anxiety at age nine. Longitudinal analysis indicated these children affected by PI also reported lower levels of happiness at age 13 with higher levels of emotional difficulties reported by their primary caregiver.

Conclusions
Children of incarcerated parents face a greater array of life challenges. PI had an association with child-reported levels of anxiety at age nine. PI also had a medium-term association on caregiver assessments of emotional difficulties of children affected as well child-reported levels of happiness over time.

Aim
To explore the association between weight and bullying; considering victims and perpetrators as two aspects of bullying, and subjective perception and objective measurement as two aspects of weight.

Methods
This study is based on the first wave of data collection from Growing Up in Ireland – the National Longitudinal Study of Children. The two-stage sample design included a sample of 910 primary schools in Ireland, from which a sample of 8568 nine-year-old children and their families was randomly selected. Analysis is based on statistically reweighted data to ensure that it is representative of all 9-year-olds in Ireland.

Results
Significantly (P < 0.001) more girls were overweight or obese (33.1%: 23.1% overweight and 10% obese) than boys (25.2%: 18.3% and 6.9%). Children who were body mass index (BMI) classified as overweight or obese were significantly (P < 0.001) more likely to be victimised when compared with children whose BMI was not classified as overweight or obese. BMI-classified thinness was not significantly associated with victimisation; however, the body image of being skinny or very skinny was significantly (P = 0.015) associated with being victimised. Bullying perpetration was not associated with BMI-derived weight classification but was significantly (P < 0.001) associated with the child’s own self-description of weight.

Conclusions
Overall body image was found to have a stronger association with victimisation and bullying perpetration than objective BMI-derived weight classification. Further research investigating the mediating role of body image in the relationship between weight, victimisation and bullying is necessary to better understand this association.

Background
Unintended pregnancy is associated with increased risk for adverse neonatal and early childhood outcomes spanning an array of indicators, but it remains unclear whether these risks hold independent of other biological, social and environmental risk factors.

Methods
This study uses data from the first wave of the ‘Growing Up in Ireland Study’, a large nationally representative cohort study of more than 11 000 infants, to examine the risk factors associated with unintended pregnancy. Adopting a staged approach to the analysis, the study investigates whether pregnancy intention influences maternal health behaviours during pregnancy independent of background characteristics, and whether pregnancy intention carries any additional risk for adverse infant and maternal health outcomes when we adjust for background characteristics and prenatal behaviours.

Results
The study confirmed that sociodemographic factors are strongly associated with unintended pregnancy and that unintended pregnancy is associated with a range of health compromising behaviours that are known to be harmful to the developing fetus. While there was little evidence to suggest that pregnancy intention was associated with adverse neonatal outcomes or developmental delay independent of other covariates, there was strong evidence that intention status had a bearing on the mother’s psychosocial health. Unintended pregnancy was associated with increased risk of depression (risk ratio 1.36 [95% confidence interval 1.19, 1.54]), and higher parenting stress (risk ratio 1.27 [95% confidence interval 1.16, 1.38]).

Conclusions
Ascertaining the mother’s pregnancy intention during the first antenatal visit may represent a means for monitoring those at greatest risk for adverse mother and child outcomes.

Background
Social inequalities in the prevalence of childhood overweight and obesity are well-established, but less is known about when the social gradient first emerges and how it evolves across childhood and adolescence.

Objective
This study examines maternal education differentials in children’s body mass trajectories in infancy, childhood and adolescence using data from four contemporary European child cohorts.

Methods
Prospective data on children’s body mass index (BMI) were obtained from four cohort studies—Generation XXI (G21—Portugal), Growing Up in Ireland (GUI) infant and child cohorts, and the Millennium Cohort Study (MCS—UK)—involving a total sample of 41,399 children and 120,140 observations. Children’s BMI trajectories were modelled by maternal education level using mixed-effect models.

Results
Maternal educational inequalities in children’s BMI were evident as early as three years of age. Children from lower maternal educational backgrounds were characterised by accelerated BMI growth, and the extent of the disparity was such that boys from primary-educated backgrounds measured 0.42 kg/m2 (95% CI 0.24, 0.60) heavier at 7 years of age in G21, 0.90 kg/m2(95% CI 0.60, 1.19) heavier at 13 years of age in GUI and 0.75 kg/m2 (95% CI 0.52, 0.97) heavier in MCS at 14 years of age. The corresponding figures for girls were 0.71 kg/m2 (95% CI 0.50, 0.91), 1.31 kg/m2 (95% CI 1.00, 1.62) and 0.76 kg/m2 (95% CI 0.53, 1.00) in G21, GUI and MCS, respectively.

Conclusions
Maternal education is a strong predictor of BMI across European nations. Socio-economic differentials emerge early and widen across childhood, highlighting the need for early intervention.

Children’s parent and teacher relationships contribute to school adjustment and achievement, yet few studies have examined interactions between these relationships, particularly for father-child relationships. Using the Growing Up in Ireland birth cohort (N = 7,507 children, 50.3% male), we examined child-adult relationship quality – rated by mothers and fathers at age 3 and teachers at age 5 – as predictors of behavioural adjustment (rated by teachers) and academic achievement (formal reading assessments and self-reported academic self-concepts) at age 9. Controlling for prior levels of problem behaviours, verbal ability, and family SES, results indicated small and comparable independent effects of children’s parent and teacher relationships on school adjustment and achievement. For mothers and teachers, moderation analyses showed a cumulative risk pattern for conflictual relationships and a compensatory pattern for close relationships. Children are likely to benefit from improving closeness and reducing conflict in adult-child relationships and interventions that involve mothers, fathers, and teachers.

Background:
Regulatory problems in infancy are associated with internalizing and externalizing symptom trajectories across childhood. It is unknown whether early screen media exposure exacerbates the association between infant regulatory problems and childhood internalizing and externalizing symptom trajectories.

Methods:
We studied 10,170 individuals from the Growing Up in Ireland ’08 cohort. Parents reported on their children’s regulatory problems at 9 months, screen media exposure at 3 years, and internalizing and externalizing symptoms at 3, 5, 7, and 9 years. Children were categorized based on their patterns of co-developing internalizing and externalizing symptoms using parallel process latent class growth analysis. Subsequently, multinominal logistic regression analyses were conducted to estimate whether screen media exposure (&gt;1 hour or &gt;2 hours) moderated the association between regulatory problems in infancy and co-developing internalizing and externalizing symptom classes across childhood.

Results:
There were four groups of children with distinct patterns of co-developing internalizing and externalizing symptoms: 1) low stable internalizing and low decreasing externalizing symptoms (N= 6236; 61.3%); 2) low stable internalizing and moderate stable externalizing symptoms (N= 2029; 20%); 3) moderate increasing internalizing and moderate decreasing externalizing symptoms (N= 1263; 12.4%); 4) moderate increasing internalizing and high increasing externalizing symptoms (N= 642; 6.3%). Presence of regulatory problems at 9 months and excessive screen media exposure at 3 years (particularly &gt;2 hours) were independently associated with increased likelihood of all classes. Excessive screen media exposure at 3 years did not significantly moderate the association between regulatory problems at 9 months and internalizing and externalizing symptom trajectories.

Conclusions:
Both regulatory problems in infancy and screen media exposure &gt;2 hours in early childhood are early risk factors for the development of internalizing and externalizing symptoms. However, the association between infant regulatory problems and childhood internalizing and externalizing symptoms appears to operate independently of excessive screen media exposure. Clinicians should be aware of, and treat, regulatory problems in infancy to prevent chronic patterns of self-regulatory problems across childhood. Further, they should discuss and advocate for family media plans with parents to help decrease the long-term negative effects on internalizing and externalizing symptoms.

Background
We examined the role of neighbourhood characteristics in explaining socioeconomic inequalities in child mental health in Ireland.Methods: Data from Cohort ’08 of Growing Up in Ireland with Wave 3 (age 5) as the baseline and Wave 5 (age 9) as the follow-up (n=8,373 for cross-sectional and n=6,349 for longitudinal analyses) were used. Socioeconomic status (SES) indicators were caregiver education, occupation, and equivalised household income. Caregiver and teacher reports of total difficulties score (TDS) from the Strength and Difficulties Questionnaire were combined with a higher score indicating more mental health difficulties. Twenty neighbourhood items rated by caregivers were grouped into five domains (neighbourhood safety, built environments, cohesion, interaction, and disorder) using exploratory factor analysis. We used linear regression to assess the associations between SES indicators, neighbourhood characteristics, and TDS, controlling for sociodemographic covariates. Mediation analysis informed the extent to which (%) neighbourhood domains explained the SES-TDS associations. Network psychometric analysis was used to identify neighbourhood items that were most strongly linked to TDS.Results: Lower SES was consistently associated with a higher baseline, follow-up TDS, and longitudinal changes in TDS. Favourable neighbourhood domains (higher safety, cohesion, interaction, lower disorder) were associated with lower baseline and follow-up TDS. Neighbourhood safety, cohesion, and disorder were also associated with longitudinal changes in TDS across waves. Four neighbourhood domains (except built environments) explained the associations between SES and baseline and follow-up TDS by 3% to 18% in separate single mediation models, and these four domains in concert explained 12% to 23% in multiple mediation models. Built environments domain predicted TDS and mediated the associations between SES and baseline TDS (5% to 11%) in urban children only. No mediation of the associations between SES and longitudinal changes in TDS by neighbourhood domains was observed. Findings from network analysis indicated that specific concerns over “people being drunk or taking drugs in public” and whether “this is a safe neighbourhood” may act as key conceptual bridges between neighbourhood characteristics and TDS.

Conclusions
Neighbourhood characteristics may explain cross-sectional socioeconomic inequalities in child mental health. Improving neighbourhood characteristics where lower-SES children live may be important to reduce SES-based inequalities in child mental health.

Background
We examined the role of neighbourhood characteristics in explaining socioeconomic inequalities in child mental health in Ireland.Methods: Data from Cohort ’08 of Growing Up in Ireland with Wave 3 (age 5) as the baseline and Wave 5 (age 9) as the follow-up (n=8,373 for cross-sectional and n=6,349 for longitudinal analyses) were used. Socioeconomic status (SES) indicators were caregiver education, occupation, and equivalised household income. Caregiver and teacher reports of total difficulties score (TDS) from the Strength and Difficulties Questionnaire were combined with a higher score indicating more mental health difficulties. Twenty neighbourhood items rated by caregivers were grouped into five domains (neighbourhood safety, built environments, cohesion, interaction, and disorder) using exploratory factor analysis. We used linear regression to assess the associations between SES indicators, neighbourhood characteristics, and TDS, controlling for sociodemographic covariates. Mediation analysis informed the extent to which (%) neighbourhood domains explained the SES-TDS associations. Network psychometric analysis was used to identify neighbourhood items that were most strongly linked to TDS.Results: Lower SES was consistently associated with a higher baseline, follow-up TDS, and longitudinal changes in TDS. Favourable neighbourhood domains (higher safety, cohesion, interaction, lower disorder) were associated with lower baseline and follow-up TDS. Neighbourhood safety, cohesion, and disorder were also associated with longitudinal changes in TDS across waves. Four neighbourhood domains (except built environments) explained the associations between SES and baseline and follow-up TDS by 3% to 18% in separate single mediation models, and these four domains in concert explained 12% to 23% in multiple mediation models. Built environments domain predicted TDS and mediated the associations between SES and baseline TDS (5% to 11%) in urban children only. No mediation of the associations between SES and longitudinal changes in TDS by neighbourhood domains was observed. Findings from network analysis indicated that specific concerns over “people being drunk or taking drugs in public” and whether “this is a safe neighbourhood” may act as key conceptual bridges between neighbourhood characteristics and TDS.

Conclusions
Neighbourhood characteristics may explain cross-sectional socioeconomic inequalities in child mental health. Improving neighbourhood characteristics where lower-SES children live may be important to reduce SES-based inequalities in child mental health.

Little is known about the link between childhood and adolescent mental health (MH) difficulties and political outcomes in adulthood. This represents an important knowledge gap in understanding early individual-level factors for future political outcomes. Using the Growing up in Ireland cohort, following 8,568 children from nine to 20 years, we examine how different MH difficulties, and co-occurrence, are associated with various political outcomes. The results show childhood/adolescent MH difficulties are important predictors of political outcomes at age 20. Adolescents with chronic co-occurring MH difficulties starting in childhood report a lower interest in politics, lower trust in politicians, are less likely to register to vote, and if voting, are more likely to vote for an anti-establishment party. Adolescents with chronic emotional MH difficulties starting in childhood report a higher likelihood of voting for an anti-establishment party, but also show a higher probability of political activism and low external political efficacy.

Psychotic experiences (PEs) are common in early adolescence and are associated with nonpsychotic psychopathology. However, not all adolescents with PEs have subsequent psychopathology, and vice versa. To date, factors mediating the relationship between PEs and psychopathology have been understudied. The aims of this study were to investigate the bidirectional relationship between PEs and psychopathology in adolescence and to investigate potentially malleable mediators of these relationships. Data from 2 waves (age 13 and 17 years) of Cohort ’98 of the Growing Up in Ireland study were examined (n = 6,206). Using KHB pathway decomposition, we investigated the following as potential mediators of the relationship between psychopathology and PEs: parent-child relationship (conflict and positive), self-concept, and child-peer relationship (alienation and trust). Supplementary counterfactual mediation and sensitivity analyses were conducted. Early adolescents with psychopathology had twofold increased odds of late adolescent PEs (internalizing problems: odds ratio [OR] = 2.03, 95% confidence interval [CI; 1.56, 2.62]; externalizing problems: OR = 1.99, CI [1.51, 2.60]). Parent-child conflict explained between 23% and 34% of the associations between internalizing and externalizing problems and subsequent PEs. Early adolescents with PEs had increased odds of late adolescent psychopathology (internalizing problems: OR = 2.01, CI [1.61, 2.50]; externalizing problems: OR = 1.70, CI [1.25, 2.31]). Self-concept alone accounted for 52% of the relationship between PEs and subsequent internalizing problems. There is a bidirectional heterotypic relationship between psychopathology and PEs. Parent-child conflict and self-concept are important characteristics that mediate a proportion of the relationship between PEs and psychopathology. Interventions targeting parent-child conflict in the context of psychopathology and self-concept in the context of PEs may assist in reducing the incidence of poorer outcomes.

Young children born preterm may be more affected by environmental influences than their full-term peers. Few studies have investigated whether such effects exist for older children and young adolescents. With participants aged 9 and 13 years, we examine whether children born preterm could be differentially affected by the quality of their relationship with their mothers and fathers. We used the Growing Up in Ireland dataset: a longitudinal sample of 8,568 children in Ireland (51.4% female, 48.6% male) and their parents. We found that parent–child conflict was consistently associated with poorer verbal, numerical, and socioemotional outcomes; in some instances, parent–child closeness was associated with better outcomes. Being born very preterm was consistently associated with negative outcomes. We found support for a diathesis-stress model of preterm birth in just one instance: children born very preterm displayed a stronger relationship between maternal conflict and increased socioemotional difficulties.

Purpose
This study examines the expressive language and speech of twins, relative to singletons, at 3 and 5 years, with the aim of determining if a twinning effect occurs during this developmental period. The possibility of twins outgrowing a twinning effect was investigated.

Method
A weighted population-based sample of 185 twins and 1,309 closely spaced singletons who participated in the Growing Up in Ireland study was analyzed. Their development was compared using a standardized expressive vocabulary assessment and parent interview at 3 and 5 years as well as teacher reports at 5 years. Causal steps mediation analyses were subsequently conducted to determine if gestation and birth weight mediated twin–singleton differences in expressive vocabulary development.

Results
Twins’ expressive vocabulary scores were slightly lower than those of singletons at 3 years, but there was no significant difference at 5 years. More parents of twins reported “a little” concern about their child’s expressive language and speech skills at 3 years, although there was no significant twin–singleton difference 2 years later. Teachers’ ratings of twins’ and singletons’ expressive language and speech skills at 5 years were comparable. Shorter gestations and lower birth weights mediated twins’ lower expressive vocabulary scores at 3 years.

Conclusions
A very small twinning effect on expressive vocabulary development exists at 3 years and is outgrown by 5 years. Twins born after shorter gestations or at a lower birth weight are at an increased risk of having less developed expressive vocabulary skills at 3 years.