Our study found that the ADHD group has a relatively higher rate of prematurity and a significantly higher rate of low birth body weight than the general population. We also demonstrated that both moderate preterm birth (gestational age between 33
weeks to 37
weeks) and low birth body weight (defined as birth body weight
g) are associated with more severe ADHD symptoms. Previous studies have concluded that very preterm children were reported to exhibit significantly more behavioral problems and cognitive disorders [16
]; however, most of these studies focused on very preterm (≤ 33
weeks’ gestation) and extremely very low birth body weight [24
]. Our data support recent literature with regard to the observation that inattention and emotional regulation difficulties affect the functioning of moderate preterm children at school age [18
The significant association between ADHD and preterm birth or low birth body weight can be explained by several mechanisms. First, some of the children suffered from intrauterine growth retardation and their less matured brain structure due to suboptimal fetal environment; these conditions may be associated with postnatal illness and later development of ADHD [28
]. Second, less favorable parent–child or family interaction with children during the first half-year of preterm or SGA (small for gestational age) [29
] may affect the neurobehavioral development of these children. Furthermore, the underlying causes of premature birth, either genetic or environment factors, may also influence or interfere with normal neuronal development and organizations, which may contribute to subsequent ADHD symptoms [30
Another interesting finding of our study was that the more severe ADHD symptoms, including inattention, hyperactivity and impulsivity, were significantly correlated with more preterm birth or lower gestational age. However, lower birth body weight was only associated with inattention, but not hyperactivity or impulsivity. To our knowledge, our study was the only one to conclude both low birth body weight and preterm birth as independent risk factors for the development of ADHD at school age. These two factors may not be correlated, because in studies of low-birth-weight (<2500
g) but term-born children, small body size at birth predicts behavioral symptoms of ADHD [28
]. These findings can be partially explained by the theory that inattention was correlated with cognition and hyperactivity or impulsivity was correlated with poor inhibition of movement. Low gestational age could affect both neurological cognition and control of movement, while birth body weight might only affect cognition. This theory deserves further prospective design with larger sample to confirm this theory.
Previous studies have demonstrated an increased risk for ADHD in follow-up studies of preterm survivors from NICUs [32
]. Even in moderately preterm children, cognitive and emotional regulation difficulties affect their functioning at school age, and a slightly lower IQ with attention and behavioral problems are found when they are compared with term-born children [35
]. In these studies, inattention problems are found in 15–25% of the moderately preterm-born children and approximately one-third of very preterm children at their school age [32
]. In addition, preterm birth carries some risk for psychiatric disorders requiring hospitalization in adolescence and young adults [33
], thus the requirement of more attention in research and secondary prevention is warranted.
A major limitation of our study is its relatively small sample size and retrospective design. We are unable to further extend our conclusion to very low birth body weight children (< or
g) and very preterm children (< or
weeks’ gestation). However, our study had excluded the cases of well-defined brain damage such as intraventricular hemorrhage, periventricular leukomalacia, or cerebral palsy from perinatal asphyxia. We also excluded children with an IQ of less than 70 on the WISC-III, and those with major mental, neurological, or physical disease, which may have resulted in the exclusion of very low birth body weight and very preterm infants from our study sample. Furthermore, our study applied ADHD questionnaires from parents and teachers rather than utilizing objective measurements of quality of life, person-to-person relationships, academic achievements, or neurocognitive tests. A further well-designed, prospective study is required to confirm our findings.
It is our hope that, by gaining a better understanding of the strong relationships between preterm birth, low birth body weight and the risk of developing ADHD, many undefined efforts can be progressed to avoid preterm birth. Further attention should be paid to these children by having child psychiatric clinics perform regular follow-ups to track their behavioral and emotional conditions.