Abstract
Attention-deficit hyperactivity disorder (ADHD) is one of the most common diagnoses among children. Biological and environmental factors contribute to the development of ADHD symptoms. Although heredity is a major determinant of ADHD, other environmental factors are suggested to increase the risk of ADHD in children. In particular, increased prenatal stress has been linked to the development of a psychiatric disorder, specifically ADHD. Therefore, the objective of this review is to analyze current literature on prenatal stress and ADHD cases to determine if there is a positive association. Conclusively, prenatal stress was found to contribute to increased ADHD prognosis in children.
Keywords: prenatal stress, ADHD
Introduction
Attention-deficit hyperactivity disorder (ADHD) is one of the most common behavioral diagnoses among young children. It is defined as a neurodevelopmental disorder characterized by hyperactivity and/or inattention that continues for at least 6 months across several situations (Rodriguez & Bohlin, 2005). The symptoms of ADHD can affect a child’s academic and social capabilities (Okano, Ji, Riley, & Wang, 2018). Although the prevalence of ADHD decreases with age, symptoms of ADHD continue throughout adulthood (Rodriguez & Bohlin, 2005).
While the causes of ADHD are unknown, both environmental and genetic factors have been implicated. Though many studies have shown ADHD to be a heritable disorder, only 75-80% of ADHD symptoms are the result of genetic factors. Studies have suggested causal factors from pre-, peri-, and postnatal environments to play a role in ADHD (Grizenko, Shayan, Polotskaia, Ter-Stepanian, & Joober, 2008). Therefore, this review primarily investigates prenatal stress as a modifiable environmental factor to prevent the development of ADHD symptoms among children.
Diagnostic process of ADHD in children
The diagnostic process of ADHD includes diagnostic tests, patient history and parent interviews. In addition, behavior scales are used by clinical psychologists to measure symptom severity of children. Clinical psychologists often use the Diagnostic and Statistical Manual of Mental Disorder (DSM) when rating a child’s behavior. In addition, behavioral scales are useful in assessing ADHD behavior at school and at home. Teacher’s ratings of children are often more accurate when analyzing external behaviors, such as aggression, because it is a disturbance to the classroom, where internalizing behaviors may not be identified as often. In addition, mothers are more likely to report inattentive behaviors rather than hyperactive behaviors (Mayfield et al., 2018). These diagnostic processes are utilized in the following studies when evaluating children; therefore, it is important to understand how ADHD is identified and assessed.
Heritability of ADHD
Heritability is defined as how much of a trait is the result of genetic influences (Grimm, Kittel-Schneider, & Reif, 2018). Many studies have found ADHD to be highly heritable. Grimm et al. (2018) have estimated heritability of ADHD to be 76%. In addition, ADHD has been found to have a stronger heritability compared to other psychiatric disorders (Grimm et al., 2018). Too, ADHD has been found to have a higher prevalence among first-degree relatives (Grimm et al., 2018). With heritability playing a partial role in ADHD, understanding the relationship between environmental factors and the development of ADHD among children is necessary.
Prenatal stress as an environmental factor
Many studies have found maternal stress to have a significant impact on a child’s development. Although several hypotheses have been formed, the mechanisms by which maternal stress causes developmental issues in children is still uncertain. Hypotheses that try to explain the onset of ADHD symptoms include, but are not limited to: the activation of the sympathetic nervous system, causing an increase in uterine artery resistance that triggers a decrease in the blood flow to the fetus; amplification of the fetal cortisol levels modifies the nervous system and dysregulates the hypothalamus-pituitary-adrenal (HPA) axis; and reduced inhibition of the frontal activity is caused by the catecholamine in the synaptic cleft in specific brain regions (Grizenko et al., 2012). Conclusively, it is biologically plausible that the development of ADHD is affected by prenatal stress.
Objectives
Three studies were selected to analyze the association between prenatal stress and ADHD. Grizenko et al. (2012) set out to determine if there was a relationship between the severity of maternal stress and the extent of ADHD symptomology in children. In addition, a different study investigated the association between prenatal maternal stress (PNMS) and ADHD behaviors in 2-year-olds (Ronald, Pennell, & Whitehouse, 2011). The final study examined whether a link between maternal psychosocial stress and the diagnosis of ADHD among children was present (Okano et al., 2018).
Methods
Current studies have suggested that maternal stress is a contributive factor of the development of ADHD in children. Longitudinal research designs have been used to assess maternal stress during pregnancy and development of ADHD symptoms in children. Research within this field have used self-report questionnaires and clinical interviews to assess children.
Participants
Participants in the studies reviewed included a disproportionate number of males and females. Studies obtained more data from males as males are more likely to be diagnosed with ADHD. The sampled population included children ranging from 2 to 12 years old.
Inclusion/exclusion criteria
In particular, one study included children who had received an ADHD diagnosis by an experienced child psychiatrist (Grizenko et al., 2012). Exclusion criteria included children with a history of Tourette’s Syndrome or children with an IQ lower than 70 (Grizenko et al., 2012). Equally important, another study had additional inclusion criteria that included English language skills and an intent on staying in the area for future follow-up assessments (Ronald et al., 2011). Conversely, in vitro pregnancies, multiple-gestation pregnancies, deliveries induced by trauma, or infants with birth defects were excluded in one of the three studies (Okano et al., 2018).
Measures used to assess symptoms of ADHD in children
Common measures that were used included the Child Behavior Checklist (CBCL), Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), and the International Classification of Diseases (ICD-9). Grizenko et al. (2012), used the DSM-IV criteria to identify ADHD symptoms in children. In addition to the DSM-IV, children were assessed using clinical interviews and evaluations, as well as the CBCL which was completed by the child’s parents. Similarly, one study used the DSM-IV symptomology list to identify ADHD and CBCL to measure child behavior during a 2 year follow up (Ronald et al., 2011). Conversely, a different study used an alternate method to diagnosis of ADHD known as the International Classification of Diseases (ICD-9) (Okano et al., 2018).
Measures used to assess prenatal stress
The following studies reviewed used multiple measures to study maternal stress. First, Grizenko et al. (2012), used the Kinney Medical and Gynecological Questionnaire to assess the stressful life events the mother encountered before, during and after pregnancy. The study analyzed the level of stress the mother experienced in each trimester to determine if stress during a particular period altered the severity of ADHD symptoms in children (Grizenko et al., 2012). A different study measured maternal stress at 18- and 34-weeks gestation by asking whether 10 stressful life events had occurred (Ronald et al., 2011). The final study measured maternal stress using three scales: the 4-item Perceived Stress Scale, experiences of daily stress during pregnancy, and the absence or presence of five major stressful happenings (Okano et al., 2018).
Results
All studies found a significant relationship between prenatal stress and symptoms of ADHD in children. In particular, Grizenko et al. (2012) discovered an association between maternal stress and symptoms of ADHD in children. Additionally, the study found a relationship between increased stress during the third trimester and high CBCL scores (Grizenko et al., 2012). Consistently, another study discovered a significant relationship between prenatal maternal stress (PNMS) and ADHD behaviors in both males and females (Ronald et al., 2011). Lastly, Okano et al. (2018) discovered single mothers that smoked during pregnancy, failed to complete high school, or had intrauterine complications during pregnancy were more likely to have children diagnosed with ADHD. A positive correlation between maternal psychosocial stresses and the likelihood of an ADHD diagnosis was also observed. Furthermore, they found males to be at higher risk of developing ADHD when the mother experienced stress during pregnancy (Okano et al., 2018).
Limitations
The following research studies demonstrated limitations that should be addressed in future studies. The use of the Kinney Medical and Gynecological Questionnaire, which relied on the mother to successfully recall the degree of stress she felt while pregnant, was utilized in one study. The dependence on the mother’s memory could have produced recall bias (Grizenko et al., 2012). Additionally, limitations included attrition bias, as underprivileged mothers were less likely to participate, and many participants failed to complete or respond to follow-up assessments (Ronald et al., 2011). Also, studies did not include a sample that was representative of the entire population. Finally, the majority of studies relied on the mother to rate the child’s ADHD behavior, which could produce a correlation error (Ronald et al., 2011).
Conclusions
All three studies demonstrated consistency in their results, in that each study found a relationship between prenatal stress and ADHD symptoms in children. Although research has found a significant relationship between the two variables, the mechanism and specific determinants remain unknown. Genetic influences, postnatal environmental influences, and elevated self-report data are possible reasons for this relationship and require further research.
Future Directions
Previous studies have demonstrated a significant association between prenatal stress and symptoms of ADHD, as mentioned above. The majority of these studies used the same measurements, such as the CBCL to identify ADHD symptoms in children. Additionally, the studies included a wide range of ages when conducting research. In total, all results demonstrated the presence of a relationship between maternal stress and ADHD symptoms in children.
Future studies should use the same scales for measuring prenatal stress and criteria for diagnosing children with ADHD for consistency, and all children should be observed by an experienced child psychiatrist. Additionally, research should gather data on the mother’s stress levels before, during and after pregnancy instead of using memories to recall the stress they experienced. Also, different developmental periods should be assessed, and participants should be followed for longer durations to further understand the relationship. All studies analyzed did not follow up with children more than once. Therefore, with ADHD being a long-term psychiatric disorder, future research should continue to measure the development and severity of ADHD symptoms. Lastly, a diverse set of participants should be examined so that the data can be generalizable to the entire population. In total, further research should address the limitations that prior research encountered, as it may impact the identification and understanding of risk factors involved in the development of ADHD symptoms.
Other environmental factors may implicate the following studies, and therefore, further research should analyze the impact of other possible factors, in addition to prenatal stress, on ADHD. Previous studies have found environmental factors, such as maternal smoking, drug or alcohol consumption during pregnancy, low socioeconomic status, and family discord to be associated with developmental cases (Freitag et al., 2012). These account for possible environmental explanations for the development of ADHD, and therefore, additional research is necessary.
Overall, current research has found a modest, yet significant, relationship between prenatal stress and ADHD outcomes in children. All studies demonstrated prenatal stress to be an important risk factor that can influence ADHD cases; however, other environmental variables may also contribute to the development of ADHD in children and should be considered in future studies. Conclusively, prenatal stress should be monitored in order to decrease the likelihood of ADHD cases in children.
References
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