Mothers who give birth through a Cesarean section could pass on stress to babies through hormones in breast milk, says a New Zealand study by an Indian-origin researcher.

The study, unveiled on Wednesday, said researchers at the University of Auckland's Liggins Institute analysed breast milk samples from 650 mothers when their babies were three-to-four months old, Xinhua news agency reported.

The findings revealed levels of the stress hormone, cortisol, which were higher in the milk of mothers who got their babies delivered by Cesarean section (C-section) or who had no partner at home.

As well as a major regulator of the body's stress response, cortisol was an important influencer of mood and growth, researcher Shikha Pundir said in a statement.

While a certain amount of stress hormones were needed to stimulate healthy development, evidence from animal studies suggested that higher cortisol in milk affected the baby's temperament.

Breast milk was recognised for its nutritional and immunity-boosting powers, but it was still unclear exactly how stress hormones affected babies' growth and development, the researchers said.

The findings underlined the importance of supporting all mothers in order to avoid the transmission of stress to babies, which could potentially have a long-term adverse impact on a baby's health.