Pregnancy and child

Caesareans and baby immunity

Children born by caesarean section are more likely to have allergies, such as asthma, because they pick up less “natural immunity” from their mother, The Daily Telegraph reported.

The story is based on a small laboratory study which investigated whether the mode of delivery affected the type of bacteria found on 10 newborn babies. Babies born by normal vaginal delivery were found to have types of bacteria that mainly resembled those found in their mothers’ vagina, while those delivered by caesarean had microbes normally found on the skin surface.

This study provides a useful contribution to our knowledge of the possible effects on babies of having a caesarean rather than vaginal delivery. However, on its own, the study is too small to offer any conclusions about the exposure of newborns to particular types of bacteria at birth, and has no implications for the long-term health of babies delivered by caesarian. Another drawback is that it did not look at any other possible differences between the mothers or their babies that may have contributed to the differences in types of bacteria, such as the use of antibiotics. As the researchers say, longer-term, larger studies are needed.

Where did the story come from?

The study was carried out by researchers from the University of Puerto Rico, University of Colorado and two research centres in Venezuela. It was partly funded by the National Institutes of Health and two charitable foundations in the US. The study was published in the peer-reviewed journal Proceedings of the National Academy of Sciences.

The media’s reporting of the study was largely accurate. However, neither The Daily Telegraph nor Daily Mail mentioned its small size, and both overstated its significance, inaccurately implying that caesarean babies are at higher risk of health problems because of the type of bacteria they are exposed to at birth. Also, the Mail’s picture caption that the “sterile op means newborns are exposed to less bacteria” is incorrect.

Both stories relied heavily on the press release published simultaneously with the study.

What kind of research was this?

This small laboratory study investigated whether babies’ mode of delivery has any effect on the bacteria present in newborn babies. The researchers say that babies born by normal vaginal delivery are exposed to a wide variety of microbes from a number of sources, including maternal bacteria, and that babies born by caesarean are not exposed to these vaginal microbes.

They say that differences in the mode of delivery have already been associated with differences in the type of bacteria in the gut of the baby. These intestinal bacteria are already known to play a role in the development of the intestine and the immune system, and the researchers suggest that similar roles are likely to be played by the presence of different types of bacteria in other parts of the body. They also suggest that a baby’s initial exposure to bacteria may serve as a “direct source” of bacteria that are either protective or harmful in newborns, and may also help define bacterial patterns as the baby develops.

The researchers used genetic sequencing to establish the type of bacteria found in samples taken from mothers and their newborn babies, just before and after birth. Although this type of study is valuable and can show patterns and associations, it cannot be used to draw any conclusions about the effects of bacteria on the health of developing babies. To be able to make any associations between the type of bacteria babies are exposed to at birth and their future health, a longer and larger study that followed babies from birth would be needed.

What did the research involve?

The researchers recruited 9 women, aged 21 to 33, and their 10 newborn babies. The women, who attended a hospital in Venezuela, were either Amerindian or Mestizo (people of mixed European and Amerindian ancestry). Four of the women delivered vaginally and five women by caesarean section, with one woman in the latter group giving birth to twins. With one exception, the mothers who delivered vaginally were not given antibiotics and had not had them in pregnancy, while the women who delivered by caesarean section had all been given antibiotics to help protect them against infection during the surgery.

One hour before delivery, swabs were taken from the mothers’ skin, mouth and vagina. Less than five minutes after delivery, swabs were taken from the babies’ skin, mouth and upper throat (nasopharynx). Rectal swabs were also taken from the babies after they had passed meconium (earliest stool). The swab samples were all frozen and transported to the laboratory, where DNA was extracted. An established method of gene sequencing was used to analyse the family and type of bacteria found.

What were the basic results?

  • The bacteria from the mothers were found to vary depending on where the sample was taken from (skin, mouth or vagina). In contrast, the bacteria harboured by the newborns were similar regardless of where in the body (skin, mouth, throat or intestines) the sample was taken.
  • As expected, babies who were delivered vaginally carried bacteria that were most similar in composition to the bacteria found in the vagina of their mothers.
  • Babies delivered by caesarean section had bacteria that were most similar to the types found on their mothers’ skin.
  • In three of the four vaginal deliveries, the babies’ bacteria were significantly more similar to their own mothers’ bacteria than bacteria in other mothers’ vaginas, suggesting direct transmission.
  • However, in babies delivered by caesarean section, the babies’ skin bacteria were no more similar to their own mothers’ bacteria than those from other mothers, suggesting that these bacteria came from non-maternal sources, such as hospital staff or fathers.

How did the researchers interpret the results?

The researchers say their results show that mothers’ vaginal bacteria provide the first natural exposure to bacteria for newborn babies, and that for babies delivered by caesarean section, the lack of vaginal exposure leads to the first bacteria resembling those found on human skin.

They suggest that the finding may in part explain why caesarean section babies seem more susceptible to certain diseases, such as MRSA skin infections, as vaginally born babies may be protected against these by their exposure to vaginal bacteria.

They also say that these initial differences may lead to longer-term differences in the patterns of bacteria in the gut and other parts of the body, which may have an effect on health. The findings, say the researchers, emphasise the need for further studies to track the development of bacteria in different parts of the body and after different modes of delivery.


This small study examined the differences in the types of bacteria found in babies delivered by caesarean section and vaginally. It is a useful contribution to the work being carried out by researchers looking at the possible longer-term effects of caesarean delivery on areas such as development of the immune system. For example, other studies have shown that in caesarean section babies, the colonisation of the gut with protective bacteria such as Lactobacillus is delayed. Other studies have suggested that caesarean section babies may be more prone to allergies and asthma, and that giving probiotics, such as Lactobacilli, from birth to age six may reduce this allergy risk in caesarean section babies but not in vaginally delivered babies. The researchers say this indicates that the composition of initial microbes may have implications for later nutritional and immune functions.

However, taken on its own, this study of 10 babies and their mothers is too small to offer any conclusions about the exposure of newborns to particular types of bacteria at birth, and has no implications for their health in the longer term. As the researchers say, longer and larger controlled studies looking at this area are needed.

Further possible reasons why the findings may have differed for babies delivered vaginally or by caesarean were not assessed. For example, the effect of antibiotics taken by the women who had caesareans was not considered. It should also be noted that the mothers and babies who took part in this study were from particular ethnic groups. Findings may differ for other ethnicities and also if a similar study was carried out in other countries which may have different obstetric care facilities and practice.
Although the caesarean rate is increasing in many countries, including the UK, in most cases the procedure is carried out because any possible risk to the mother or baby as a result of the caesarean is outweighed by the safety benefits of the procedure for both the mother and baby.

NHS Attribution