Delivery by C-section has been associated with increased allergy, diabetes and leukemia risks, said Dr. Mikael Norman, a pediatric specialist at the Karolinska Institute in Stockholm. Although the underlying cause is unknown, our theory is that altered birth conditions could cause a genetic imprint in the immune cells that could play a role later in life.

Norman and his colleagues collected blood samples from 37 infants just after delivery and three to five days after birth to assess the degree of DNA-methylation in the white blood cells, which are a key part of the immune system. In DNA-methylation, DNA is chemically altered to activate or turn off genes in response to changes in the external environment.

According to the report, higher DNA-methylation rates were found immediately after delivery in 16 infants born by C-section than the 21 infants born by vaginal delivery. Three to five days later, DNA-methylation was similar in both the groups.

The researchers felt that further research is required to determine why infants born by C-section have higher DNA-methylation rates after delivery.

Animal studies have shown that negative stress around birth affects methylation of the genes, and therefore it is reasonable to believe that the differences in DNA-methylation that we found in human infants are linked to differences in birth stress, the researchers wrote.

We know that the stress of being born is fundamentally different after planned C-section compared to normal vaginal delivery, they explained. When babies are delivered by C-section, they are unprepared for the birth and can become more stressed after delivery than before. This is different [from] a normal vaginal delivery, where the stress gradually builds up before the actual birth, helping the baby to start breathing and quickly adapt to the new environment outside the womb.