Mothers who have had a c-section or are raising a child on their own have significantly higher levels of stress hormones in their breast milk, according to new research.

Researchers at the University of Auckland's Liggins Institute have found stressed mums, triggered by a traumatic birth experience or not having the support of a partner living at home, had up to four times as much of the stress hormone cortisol in their breast milk.

The research led by Professor David Cameron-Smith and carried out by Shikha Pundir analysed 650 breast milk samples from mothers in Finland when their babies were aged between 3 and 4 months old.

Cameron-Smith said while all breast milk contained cortisol, the levels were significantly higher in stressed mums.

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"This points to the fact that we need to be very committed to making sure we can do everything we can for mothers in difficult circumstances. It ... shows how important it is for mothers to look after their own health and to try as best they can to cope and get by those first few critical months."

The institute was now extending its research to find out what impact higher levels of cortisol had on babies' health and was using breast milk collected from mothers in Western Australia.

While there was no research on human babies, evidence from animal studies indicated higher levels of cortisol in mothers' milk affected babies' temperaments. Cameron-Smith said when the animal was stressed so was its baby and this manifested into crying and sleeping problems.

However, Pundir said stress hormones were not bad as a certain amount was required to stimulate healthy development - but the question was how much.

"Our study shows that maternal biological and social environment can significantly modulate milk composition ... But more research is required to know if there are any long-term effects of these secondhand hormones on infant physiology."

Factors that were not linked to stress hormone levels included a mother's age, BMI, weight gain during pregnancy, gestational diabetes, educational level and how many other children they had.

New mother Marcelli Coronet, 35, said she always wanted to breastfeed her daughter Martina Coronet De Mattos, who is now 5 months old, because of the nutritional benefits. She said the first few months were tough for anyone trying to breastfeed and she was not surprised there were more stress hormones found in breastmilk of mums who were put under even more pressure.

Le Leche League NZ co-director Janine Pinkham welcomed any research which recognised the need for new mothers to be properly supported.

"Maternal stress can impact on the response of a mother to her baby, resulting in increased stress in the baby ... In addition, there are relaxing hormones in breast milk that help to calm both the mother and baby. Breast milk is certainly an amazing food and finding out more about it could be helpful in ensuring adequate support mechanisms for new mums."