Topic Editor: Vivette Glover, MA, PhD, DSc, Imperial College London, United Kingdom
Maternal stress in humans
The core of the research on the connection between maternal stress and child development focuses on the lasting effect of prenatal exposure to stress. Although the placenta serves as a barrier to protect the baby against harmful substances, some hormones such as cortisol released in times of stress, can cross the placenta and alter the intrauterine environment. A few studies have detected small associations between prenatal stress and low birthweight and prematurity.
Generally, prenatal stress has been associated with a number of detrimental outcomes, including cognitive, language, behavioural, emotional, and neurodevelopmental problems. For instance, children whose mothers experienced high levels of anxiety during pregnancy are twice as likely as other children to experience difficulties of a behavioural or emotional nature persisting in early adolescence. However, the connection between the presence of elevated cortisol in the amniotic fluid and poorer cognitive development disappear when children experience sensitive and optimal parental care after birth.
The impact of stress also varies as a function of gestational period. Whereas the risk of developing schizophrenia later on in life is associated with acute stress during the first trimester, Attention deficit hyperactivity disorder (ADHD) is more common among children of expecting mothers who are exposed to stress during late pregnancy. The amount of stress experienced by the mother is also important because mild stress may be related to positive outcomes on children’s motor and cognitive development.
Babies of mothers from poor economic background are particularly at risk for the harmful effects of stress due to the frequency and uncontrollability of the stressors this population experiences. However, because stress is also associated with unhealthy behaviours such as smoking and alcohol consumption, it is often difficult to isolate the effects of stress itself and other negative events.
Given the potential risk that prenatal stress poses for child development, there is a pressing need to find ways of establishing the causal connections in humans so that successful and well-targeted interventions can follow. For instance, research should compare siblings exposed and not exposed to prenatal stress to tease apart the role of genes and parenting on the association between stress and postnatal development. The role of coping strategies in successfully reducing prenatal stress and later developmental problems is another encouraging avenue of research. Research designs should also include clinical trials (for both medications and behaviour therapies), biological and psychological measures of prenatal stress, and objective assessments of child development while ensuring adequate control of maternal stress after birth.
Animal research on maternal stress
The most conclusive evidence for establishing a causal link between prenatal stress and child development has been generated by studies on animals, primarily rodents (rats and mice) and non-human primates (monkeys and apes), in which prenatal stress can be experimentally manipulated.
Research with primates found that stress during pregnancy was related to slightly lower birth weight, as well as attentional, behavioural, and motor problems in the offspring. The negative effects of prenatal stress on birth weight and behaviour were strongest when paired with prenatal alcohol consumption. In addition, attentional and learning problems observed in the postnatal period tended to persist during adolescence. Although chronic stress in the second half of gestation was related to some of these deficits, stress overall was more detrimental in the early stages of pregnancy.
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