Pregnancy and child

Caesarean birth link to asthma lacks proof

‘Babies delivered by caesarean section at higher risk of asthma and allergies’, the Mail Online reports. This rather bold claim is based on a tiny, genetic study. A link between caesareans and asthma or allergy cannot be proven from the research this story appears to be based on.

In the study, researchers looked at bacteria in the gut of 24 babies to see if there were differences between babies who had been delivered naturally (vaginally) and those delivered by caesarean section. They also looked at whether they were breastfed during their first three to four months of life.

Researchers wanted to test the theory that natural delivery and breastfeeding helps stimulate the production of healthy bacteria in the gut.

They confirmed that, in their small sample, the caesarean-born babies had lower ‘richness’ and diversity of bacteria species compared to naturally born babies. A similar pattern was not found when breastfed and formula-fed babies were compared – formula-fed babies actually had higher ‘bacterial richness’, which the researchers say is consistent with previous studies.

This is of potential interest as there is a general consensus that gut bacteria has been linked to an increasing number of diseases, including type 1 diabetes, obesity, cancer, allergies, and asthma.

However, this was such a small study, involving single measurements taken at only one point in time, that it cannot prove any link between caesarean sections, breastfeeding, gut bacteria and the likelihood of developing a long-term disease.

The researchers note these findings are part of ongoing research and that future reporting will provide more information.

Where did the story come from?

The study was carried out by researchers from the Universities of Alberta and Toronto, Canada, among other institutions. It was funded by the Canadian Institutes of Health Research and supported by AllerGen NCE, the Killam Trusts and Alberta Innovates – Health Solutions. The study was published in the peer-reviewed Canadian Medical Association Journal.

The results of the study were exaggerated in the Mail Online’s headline. The researchers have just taken a first step in attempting to establish a link between caesarean section and disease, but the headline implies they are already at the finish line.

Reassuringly, once you get past the headline, the study was covered appropriately. The print edition of the Daily Mail carries the possibly better headline 'Caesarean babies "lack protection of vital bugs"'.

What kind of research was this?

This was a cross-sectional study looking at the gut bacteria of babies, and how this differed depending on whether they were delivered naturally or by caesarean section, and whether they were breastfed or fed formula during their first three to four months of life.

The researchers say that the development of bacteria in the gut in the early part of a person’s life is poorly understood. However, the design of this study means that it arguably adds little to that understanding. It only examined the gut bacteria of an extremely small sample of babies at one point in their life and can tell us little else about the causes of these bacterial levels, or how they related to longer-term health outcomes.

It could be the case that this was a ‘proof of concept’ piece of research – to see if the genetic sequencing techniques used in the study could provide useful results.

What did the research involve?

Researchers included 24 healthy babies from the wider national Canadian Health Infant Longitudinal Development (CHILD) study, which is reported as being representative of the infant population in Canada.

The researchers obtained information on the type of delivery (natural or caesarean) from medical records as well as use of antibiotics, group B streptococcus infection (a bacteria that can cause severe infection in newborns), and whether there was early (premature) rupture of the amniotic membrane surrounding the baby (which can increase the risk of infection). The mothers of the babies were asked to report on their baby’s diet during the first months of life, and whether this was categorised as exclusively breastfed, partially breastfed or not breastfed. They were also asked about any medication use by the mother or baby.

Samples of the baby’s faeces were then collected when the infant was three to four months old and researchers examined the gut bacteria from these samples using specialist DNA sequencing techniques.

What were the basic results?

Of the 24 babies included in the researchers’ analysis, 25% were delivered by caesarean section (six babies) and 75% were delivered naturally (18 babies). By the time the babies were three to four months old:

  • 42% were exclusively breastfed (10 babies)
  • 21% were partially breastfed (five babies) – supplemented with formula
  • 38% were not breastfed (nine babies)

Exclusive breastfeeding was more common among infants born naturally (44%) compared to caesarean delivery (33%).

The main findings of the study were:

  • Compared to babies who were delivered naturally, caesarean delivered babies had significantly lower amounts of a type of ‘good’ bacteria. 
  • Compared with babies who were breastfed, those who were not breastfed had significantly higher amounts of ‘bad’ gut bacteria.
  • The amount of a particular ‘bad’ bacteria called Clostridium difficile was significantly lower among exclusively breastfed babies than among babies who received formula. This finding was not affected by the type of delivery.
  • Formula-fed infants had increased ‘richness’ and diversity of bacteria species compared to breastfed babies.
  • Meanwhile, caesarean delivered babies had the lowest ‘richness’ and diversity of bacteria species compared to naturally born babies.

How did the researchers interpret the results?

The researchers conclude that bacteria in the gut can be influenced by parent and physician decisions regarding the type of delivery and infant diet. They say that further research is needed into the determinant of the gut bacteria and any associated health outcomes.

In discussing the research findings, co-author Dr Anita Kozyrskyj, of the University of Alberta, is reported as saying, ‘Our findings are particularly timely given the recent affirmation of the gut microbiota as a “super organ” with diverse roles in health and disease, and the increasing concern over rising caesarean delivery and insufficient exclusive breastfeeding.’


Overall, this research provides some information on the amount of particular bacteria species present in the gut in the first few months of life of an extremely small number of babies. The study does not provide any evidence that the mode of delivery or feeding pattern was the cause of the bacterial levels measured. Neither does the study provide any evidence that being born by caesarean delivery leads to developing asthma later on in life, as the headline in the Mail Online suggests.

The researchers note that these findings are part of an ongoing study and future reporting will provide more information. There are some limitations to this study some of which are noted by the authors, including:

Study design

Researchers based their analysis on one measurement taken at one point in time (when the baby was three to four months old). The researchers note that previous studies suggest gut profiles vary widely in the first year of life. A more comprehensive study would have taken measurements at a number of time points of the child’s life, to determine if any changes in gut bacteria occurred. However, even then it would be difficult to pin an exact cause on the levels seen which are likely to be influenced by multiple factors.

Study size

Only 24 babies were included in the study. A study of this size is too small to reliably detect any differences between natural and caesarean deliveries, and formula and breastfed babies, and even less so to detect any differences according to type of caesarean delivery (emergency vs. elective) or brand of infant formula, for example.

Overall, no conclusions can be drawn from this small study of 24 babies.

NHS Attribution