A new study suggests the children of women who are depressed during pregnancy succumb to depression more likely during their teenage.

Researchers at Bristol University have suggested that stress hormone cortisol, which is higher in depressed mothers, may affect the developing brain of the foetus in the womb as they are able to cross the placenta.

The research has been carried out by Rebecca Pearson, research epidemiologist at Bristol University's school of social community medicine.

Depression during pregnancy may affect a baby through stress hormones that move across the placenta, Rebecca Pearson and her colleagues said.

This goes against the suggestion of some researchers that depression is only important if it continues past the end of pregnancy and impacts parenting.

"It should be treated during pregnancy, irrespective of if it continues during birth. It's as important during pregnancy," Pearson said.

Further, the study also showed that postnatal depression in the mother was a risk factor for children's depression in late adolescence, but only in mothers with low educational attainment.

Pearson also said women should do what they can to put their own mental health first during pregnancy, and know that by doing this they are also looking after their baby.

The study is published in JAMA (Journal of the American Medical Association) Psychiatry.