A world-first study aims to reveal the health benefits that conventionally-born babies may get in early life - and which babies born by Caesarean-section miss out on.

Babies born by C-section have a greater risk of becoming overweight and developing obesity, asthma, eczema and other allergic disorders in childhood.

And the rate of C-section births in New Zealand is rising, with nearly half of births in some main centres involving the procedure.

Scientists say it's possible that the bacteria that babies are normally exposed to during vaginal birth play a role in establishing normal gut bacteria in infants - and in doing so, assist in the development of their auto-immune system.

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"For millennia we have viewed bacteria as essentially bad for us, causing infections and death," explained Celia Grigg, a midwife and a research fellow at Auckland University-based Liggins Institute.

"However over the past 15 years studies in mice suggest that our gut bacteria - gut bugs or gut microbiome - are essential for normal health and wellbeing."

Not all bacteria are bad for us, and a healthy balance of gut bugs are important for good health.

Conversely, disordered gut bugs are associated with a wide range of serious diseases that include obesity, diabetes, asthma and eczema, inflammatory bowel disease, autism and Parkinson's disease.

Today, C-sections are an increasingly common mode of delivery, accounting for around a quarter of New Zealand births - and as many as 40 per cent of births at Auckland City Hospital.

Studies of children born by C-section have found that they have a 20 per cent increased risk of several important childhood conditions such as obesity, asthma and eczema.

Grigg pointed out that babies were born with few if any bacteria in their gut.

The initial bacteria that establish a baby's gut bugs came from mothers' bacteria in and around her birth canal.

"Thus it is proposed that babies born by C-section have their gut bugs established from exposure to environmental bugs which are less healthy," she said.

"Thus the less healthy gut bugs lead to later obesity, asthma and eczema."

Findings from previous studies suggest that what's commonly called "vaginal seeding" might help babies born by caesarean establish normal gut bacteria.

One such study was well publicised and led to people trying vaginal seeding themselves, despite a lack of high quality evidence on benefits and risks.

A new trial, being led by the Liggins Institute, aimed to fill this gap.

The ECOBABe study (Early Colonisation with Bacteria After Birth) involved 40 sets of twins born by C-section in Auckland over the next 15 months.

In each set, one twin only will swallow an oral infusion containing bacteria swabbed from the mother.

The study also included a separate "control" group of mothers having only one baby vaginally.

Researchers planned to measure the range and number of gut bacteria in all the babies by analysing their stools.

They would also compare the gut bacteria of C-section babies and vaginally born babies, to see whether there are differences, along with babies' weight, height and body fat composition.

"If the treatments with the mothers' microbiome actually improves the health and wellbeing of the children in terms of obesity and asthma, it's going to be a simple thing to scale up and do," said the study leader, Professor Wayne Cutfield.

Grigg added that many women and midwives were aware that caesarean section disrupted the normal process, and when it is necessary, they want to minimise negative impacts for babies, where possible.

"People have said to me 'it just makes sense, even if it sounds a bit weird or artificial'."

But she added there was no evidence to date that the practice was unsafe.

"The babies would have been exposed to the same bacteria for a much longer period if they had been born vaginally," she said.

"Although, as this is research, we are doing extra screening for women in the study, as some have expressed concern about this aspect."

The research team was recruiting twins for the trial with the help of local obstetricians and midwives.

Auckland women interested in learning more about the study could visit the study page or call/text 027 606 5140.

BY THE NUMBERS

• Latest provisional figures from the Ministry of Health show there were 16,423 C-section births in this country last year - 25 per cent of all live births.

• C-section births have reached 40 per cent in Auckland City Hospital, a large referral centre.

• Studies of children born by C-section have found that they have a 20 per cent increased risk of several important childhood conditions such as obesity, asthma and eczema.