By Emma Young
Over the past few decades, it’s become clear that experiences even before birth influence later psychological wellbeing. A mother’s stress levels during pregnancy have emerged as a key influence. Greater stress seems to programme her child to “expect” a difficult environment, and so to be more sensitive to potential threats — and more vulnerable to developing an anxiety disorder. It’s uncertain, though, whether adversity earlier in life affects stress levels during pregnancy, and so might impact the child’s sensitivity to stress. So Cassandra L. Hendrix at New York University and colleagues set out to investigate.
For their study, reported in the Journal of Psychopathology and Clinical Science, the team recruited 217 Black American mother-infant pairs from two hospitals in the Atlanta, Georgia area. To try to ensure a socioeconomically diverse sample, one hospital was public and the other private.
At around the end of the first trimester, each woman reported on their perceived stress, anxiety and depression. The team combined their answers to generate single individual scores of early pregnancy prenatal distress. The women also reported on their lifetime experience of race-based discrimination, and on any traumas, such as abuse or divorce, during their childhood. When they were in their third trimester, they completed the stress, anxiety and depression questionnaires again, generating late pregnancy distress scores.
After their babies were born, the team measured the infants’ stress sensitivity in two ways.
Firstly, they used a behavioural measure, called the NICU Network Neurobehavioural Scale. For this, a trained examiner induces mild stress, and monitors the infant’s responses. This was done when the babies were just two weeks old.
The second measure was hormonal. The team measured levels of cortisol (which is produced as part of the stress response) in saliva samples gathered from a subset of the babies when they were aged between three and six months.
Their analysis of all the data led to a few main findings.
Greater maternal distress in late pregnancy, specifically, was linked to poorer scores on one component of the behavioural measure: attention. This was a measure of the infant’s ability to respond to, attend to, and track objects in the environment. Earlier work has found that newborns with poorer scores on this grow into toddlers who are less able to regulate their emotions — which makes them more reactive to potentially stressful experiences. (In line with these findings, the researchers found that lower attention at two weeks was linked to greater cortisol responsivity at 3-6 months).
The researchers also found that women who’d experienced more race-based discrimination during their lives also reported higher levels of late pregnancy distress. Greater experience of race-based discrimination was, then, indirectly linked to lower newborn attention scores
“This pattern of results highlights late gestation as a sensitive period for the intergenerational effects of stress exposure on developing attentional abilities,” the team writes.
But the team also found that, even when prenatal distress scores were taken into account, those who’d reported more race-based discrimination before pregnancy and/or more childhood trauma were more likely to have babies with atypical cortisol levels.
This suggests that however stressed a pregnant woman feels, stressful experiences from before her pregnancy can also have their own impact on her child’s sensitivity to stress. This might happen through changes to the way particular genes are expressed in the mother, affecting her own cortisol levels and then influencing her foetus as it develops.
It isn’t easy to draw very firm conclusions from this kind of research. That’s because it’s hard to control for every possible factor that might influence a child’s stress sensitivity. For example, a woman who is stressed late in pregnancy is likely to still be stressed after her baby is born — and this will affect the infant, too. It’s also certainly worth pointing out that the link reported between maternal distress in late pregnancy and poorer scores on attention was only marginally statistically significant.
However, the team is far from alone in finding that stress has a “cascading” impact across generations. And as they write, work that enhances our understanding of the specific contributors to this process “is integral in illuminating how adversity becomes biologically embedded and ultimately increases psychological risk across generations.”