Their findings highlighted the underappreciated link between the mental health of pregnant women and the health of their placenta, the researchers from the Developing Brain Institute at Children's National Hospital, US, said.

They also said that the long-term neurodevelopmental impact on their children was under investigation.

Their study is published in the journal Scientific Reports.

Participants for the study were recruited at Children's National as part of a clinical trial aimed at reducing pregnant women's elevated stress levels during the pandemic. 165 women pregnant before March 2020 and 63 women who became pregnant during the pandemic were taken on for the study.

Those pregnant during the pandemic were not knowingly exposed to COVID-19, and they collectively scored significantly higher on questionnaires measuring stress and depression.

"During the pandemic, mothers were exposed to a litany of negative stressors including social distancing, fear of dying, financial insecurity and more," said Catherine Limperopoulos, chief and director of the Developing Brain Institute, which led the research.

Comparing the magnetic imaging resonance (MRI) data of the women who were pregnant before March 2020 to that of those who became pregnant during the pandemic, the scientists found key differences in how the placenta grew and developed among women pregnant during the pandemic. Changes in placental development were also found to be associated with the infant's birth weight at delivery.

Importantly, the researchers found these changes to be connected to maternal stress and depression symptoms. The findings suggested, they said, that the disturbances measured on placental development in the womb may influence the placenta's ability to support fetal health and wellness.

"We now know that this vital organ was changed for many mothers, and it's essential that we continue to investigate the impact this may have had on children who were born during this global public health crisis," said Limperopoulos.

Previous studies have shown that the placenta adapts to negative changes in the maternal environment and mental health status, and disruptions in placental function impact infant brain development and children's neurobehaviour and temperament.

"When identified early, maternal stress is a modifiable risk factor that can be treated with psychotherapy, social support and other personalised, evidence-based interventions," said Limperopoulos.

"We look forward to continued research in this area to better understand the mechanisms behind these biological changes and the needs of mothers and children who are born during pandemics, natural disasters and other significantly stressful events," said Limperopoulos.