Maternal prenatal stress has a bigger impact on female infants compared to male infants, according to a study recently published in Physiology & Behavior.

Scientists believe that unstable maternal mood during pregnancy causes an increase in cortisol, the body’s stress hormone. Excess cortisol crosses the placenta and can modify the developmental trajectory of the foetus. A multitude of studies have highlighted the effect of increased maternal cortisol and unfavourable outcomes, such as lower birth weight and shorter gestation period.

There have also been links to heightened maternal cortisol and emotional problems including anxiety, depression and psychopathy in childhood. However, the effects of maternal cortisol are not clear cut. Other studies have found no link to cortisol and developmental problems and some studies have found that the effect of cortisol on development depends on the point in the pregnancy at which the sample was taken.

In order to prevent and treat mental health difficulties later in life it is crucial to understand the underlying mechanisms by which cortisol influences a developing foetus. A team of scientists led by Elizabeth Braithwaite (University of Reading, UK) carried out a study that investigated how prenatal stress impacts on development during infancy.

In the study, 1,233 mothers collected saliva samples at 32 weeks gestation at three different time points in the day: on waking, 30 minutes after a walk and in the evening. Infants were assessed at 5 weeks old for negative emotionality.

The results showed that maternal cortisol samples taken upon waking correlated with increased irritability in female infants and decreased irritability in male infants. But, cortisol sampled later in the day showed no effect on irritability in both sets of infants. The findings of this study support current evidence that there are sex differences in the way foetuses are programmed to develop. A similar study found that female infants develop a more fearful temperament after being exposed to prenatal stress.

Overall, the study highlights the important role that maternal cortisol has on emotional development in infancy. The results of this study combined with previous research suggest that maternal cortisol has sex-dependent effects on developing infants. The results from this study will be important for future research that focuses on the development of prevention and intervention strategies against the effects of prenatal maternal stress.