Trajectories and predictors of developmental skills in healthy twins up to 24 months of age
Introduction
Developmental skills can be roughly categorised as cognitive, communicative, social and emotional, and psychomotor development. Children are expected to have reached developmental milestones in each of these categories within a certain age window (Anderson, Northam, Hendy, & Wrennal, 2005). Developmental delays have been related to a variety of biological, economic and lifestyle factors. Firstly, children need to have access to resources that allow them to have the opportunity to learn necessary skills; these resources may be lacking in families of lower socioeconomic status (SES). Indeed, low SES has been found to be a risk factor for developmental and behavioural problems (Bradley and Corwyn, 2002, Laucht et al., 1997, Pike et al., 2006). Further, biological factors that have been related to delays in cognitive skills are mostly ante- and perinatal, such as lower birth weight (Datar and Jacknowitz, 2009, Jeng et al., 2000), shorter gestations (Hediger et al., 2002, Sullivan and Margaret, 2003, Thun-Hohenstein et al., 1991) and Apgar scores lower than 7 (de Moura et al., 2010, Odd et al., 2008). Maternal age could be a mediator between perinatal outcomes and developmental skills in the following ways: very young maternal age (<15 years) (Mousiolis, Baroutis, Sindos, Costalos, & Antsaklis, 2013) has been associated with an increased risk for preterm birth, while older maternal age (>40 years) has been related to shorter gestations and low birth weight (Lung, Shu, Chiang, & Lin, 2009). Lung et al. (2009) also reported that older mothers were more likely to opt for assisted reproductive treatment, particularly in vitro fertilisation (IVF), which has been related to preterm birth and low birth weight (Hvidtjorn et al., 2009). In a recent review on the developmental consequences of IVF, Hart and Norman (2013) found no evidence that children conceived through IVF were at higher risk for developmental delays. However, they reported several studies that suggested any differences between IVF- and naturally conceived children could be due to factors related to maternal age and preterm birth.
Not all domains are equally affected by perinatal and maternal factors. For instance, maternal age has been more often related to cognitive development and behaviour (Fergusson and Lynskey, 1993, Ketterlinus et al., 1991), while motor skills seem more sensitive to perinatal morbidity (Sullivan & Margaret, 2003). Gender differences in behavioural and cognitive development may be observed as early as infancy, but become more apparent from childhood onwards (Alexander & Wilcox, 2012).
Even though most children seem to catch up on developmental skills in early (Gasson and Piek, 2003, van Haastert et al., 2006) and middle childhood (Thorpe, 2006), a major consequence of developmental delay is the higher use of special school services (Sullivan & Margaret, 2003) and higher risk of subsequent emotional and behavioural problems (Emerson & Einfeld, 2010). Furthermore, children with general developmental delay (delays in at least 2 developmental domains) show less activity participation compared to their peers, which can be partly explained by impaired social and motor skills (Leung, Chan, Chung, & Pang, 2010). Psychomotor skills have also been argued to change the way an infant interacts with its environment, which could be important for the development of communicative skills (Iverson, 2010). Moreover, fine motor skills in infancy are strong predictors of later intellectual functioning (Thun-Hohenstein et al., 1991) and general academic achievements at school age (Sullivan & Margaret, 2003). Furthermore, earlier attainment of gross motor milestones have been associated with better executive performance at school age (Piek, Dawson, Smith, & Gasson, 2008) and learning to stand at an earlier age has been associated with better cognitive categorisation in adulthood (Murray et al., 2006), which in turn has been associated with reasoning ability (Salthouse, 2005) and specific areas of language development (Gopnik & Meltzhoff, 1987).
Tools to assess these developmental milestones include the Bayley Scales of Infant Development (Bayley, 2006), Wechsler Intelligence Scales for Children (Wechsler, 2003) and Griffiths Mental Development Scales (McLean, McCormick, & Baird, 1991). There are also developmental screening tools, which give a general overview of a child's cognitive abilities and do not take as much time to administer, as they are often questionnaires that can be completed by parents. Examples of such screening tools are the Ages and Stages Questionnaire (Squires, Bricker, & Potter, 2009) and the Parents Evaluations of Developmental Status (Glascoe, 2010). Generally, normative groups for developmental assessments consist of healthy singletons. Sutcliffe and Derom (2006) addressed the question whether twins can be directly compared to singletons. Even when otherwise physically healthy, twins are likely to be born at earlier gestations (average of 37 weeks), with lower average birth weights and have older mothers. All of these factors have been directly or indirectly related to developmental delay and later cognitive skills as outlined above. Twins have been previously found to have lower intelligence quotients (IQ) than their singleton peers up to adolescence (Cooke, 2010, Drillien, 1969, Ronalds et al., 2005). Furthermore, it has been suggested that development may vary between types of twins, whereby dizygotic twins attain gross motor milestones at an earlier age than monozygotic twins (Brouwer, van Beijsterveldt, Bartels, Hudziak, & Boomsma, 2006).
The majority of previous studies have mainly focused on risk factors for developmental delay after the first 2 years. However, to our knowledge, no previous research has been conducted into the development in the first 2 years of healthy twins, who were not born extremely small, small-for-gestational-age or very premature. Therefore, this study serves to investigate the association of perinatal outcomes (birth weight and Apgar score) and maternal age with developmental skills in the first 2 years of life in healthy infant twins. Additionally, we used a developmental screening tool to compare the development of healthy twins between the ages of 3 and 24 months with the standardised scores based on normally developing singletons.
Section snippets
Participants
Participants were part of the Birmingham Registry for Twin and Heritability Studies (BiRTHS) (Krone et al., 2006), a multiple birth registry, which recruits eligible families from three large hospitals in Birmingham, United Kingdom. Mothers of twins in England are under the care of consultants in hospitals. We recruited these mothers, between 12 and 28 weeks of gestation, in the period of 4th August 2008–15th July 2011. Following the World Health Organisation standards for a viable live birth,
Participants
Overall, 53% of the children were male. A third of the children were conceived through fertility treatment, of which 26% were through IVF. The median gestational age at birth was 37 weeks (range: 26–39 weeks). Mean birth weight was 2.3 kg (kg; SD = 0.55), ranging from 0.94 to 3.5 kg. Two infants weighed <1 kg at birth and 11 infants weighed between 1 and 1.5 kg. The median Apgar score at 5 min was 9 (range: 4–10). Mothers’ mean age at delivery was 31.9 years (standard deviation [SD] = 5.51, range: 20–41
Discussion
In this study, we found that healthy twins show signs of developmental delay in communication, motor, problem solving and personal-social skills during the first year of life, even when adjusted for prematurity. These developmental delays remained even when three very preterm twin pairs were excluded (results not shown), and could therefore not be due to the inclusion of children born <30 weeks. In contrast to other studies (Asbury et al., 2008, Rutter and Redshaw, 1991, Rutter et al., 2003),
Limitations
We used the ASQ-3 in this study, because they are easy parent-completed questionnaires and require very little time to complete. However, the questionnaires are limited by a ceiling effect, where children cannot score higher than ‘normal development’ on each subscale. The use of other (observational or interactive) assessment tools is necessary to confirm the current findings.
Although sample size varied at each age, there were no significant differences in average and range for birth weight,
Conclusion
Although the data in the current study did not allow for longitudinal analyses, healthy twins did score below the normal range on current singleton norms for cognitive, communicative, social and emotional, and psychomotor development at every follow-up in the first year of life. Considering the previous literature on the development of twins compared with singletons, our finding that twins are delayed in early infancy seems to be reasonable. Whether these results are comparable to preterm
Financial disclosure
This research was funded by the Birmingham Children's Hospital Research Foundation [BCHRF102f].
Conflict of interest
The authors declare no conflicts of interest.
Acknowledgements
The authors would like to thank the School of Psychology and Speech Pathology at Curtin University for hosting CN, while conducting this research. We are also grateful to all of the BiRTHS families who participated in the study, and hospital staff who have helped in recruitment and data collection. Ethical approval for this study was obtained from the Birmingham North, East and Solihull Research Ethics Committee in 2006 [06/Q2706/87].
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