A correlational study recently published in the Journal of Allergy & Clinical Immunology revealed an interesting connection between birth delivery type and allergies – children born by C-section appear to have a higher risk of food allergies than those who've gone through a natural birth. Yet, the complete opposite is true for children born preterm.

Scientists came to the conclusion after analyzing the allergy history of more than 1 million children born between 2001 and 2012, all of whom had been listed on the Swedish Medical Birth Register or National Patient Register. After adjusting for sex and maternal factors (age at delivery, BMI, country of birth, smoking, and asthma), the researchers compared birth type (vaginal or cesarean, which was divided further into elective or emergency cesarian) and the development of food allergies in childhood.

Over 13 years, 2.5 percent of those children (26,732) was diagnosed with a food allergy, but the risk was quite a bit higher among those born by cesarian. Those who had undergone an elective cesarian were 18 percent more at risk, while those who had been through an emergency cesarian delivery were at a 21 percent increased risk.

In contrast, those who had a very preterm birth (ie before 32 weeks) had a 26 percent lower risk of developing a food allergy compared to average. There was also some slight variation between moderately preterm (4 percent decreased risk for those born between 32 and 36 weeks) and post-term (1 percent increased risk), but these weren't statistically significant.

"This positive association strengthens the theory that exposure to vaginal microflora might reduce the risk of offspring atopic manifestation," the study authors propose.

The researchers were also able to pinpoint the median age of first diagnosis (1.6 years) – though some were diagnosed at just a few months and at least one did not receive a diagnosis until they were 12.8 years – as well as identify other factors that might put a child at risk. For example, female children and children with mothers who had asthma, pulmonary disease and/or a Swedish background were more likely to develop food allergies.

The size and length of the study provide strong evidence in favor of the vaginal microflora hypothesis but the results are correlational, not causal. As the authors note, there may be confounding factors like breastfeeding rates, levels of antibiotic use, and nutrition advice affecting the data. However, as they also point out, their conclusions do seem to support previous research into birth type and the development of the immune system.

"It has previously been implied that mode of delivery is a significant determinant of postpartum adaptation of the immune system," the authors explain. "Cesarean delivery seems to delay and alter the development of the offspring's immune system, subsequently increasing the risk of atopic disease."

And it's not just food allergies. Cesarian births have been linked to various health issues, including obesity, asthma, and diabetes. Some have even gone as far as to suggest it may be altering human evolution.