Dr. Mette C. Tollanes, of the University of Bergen, in Norway, and colleagues looked at the modes of delivery among more than 1.7 million single births reported to the Medical Birth Registry of Norway between 1967 and 1998. They used registry data from Norway's National Insurance Scheme to determine the number of children who, through the age of 18 or the year 2002, developed severe asthma.
Between 1967 and 1998, asthma risk was about 50 percent higher among children born by C-section compared with children born spontaneously and vaginally, the researchers report. They found 19 percent increased risk among children born by vaginal delivery requiring the use of instruments (forceps or vacuum).
From 1988 through 1998, when the birth registry designated between emergency and planned C-section deliveries, unexpected differences emerged. Compared with spontaneous vaginal deliveries during this period, emergency C-sections carried higher asthma risk (59 percent) than planned C-sections (42 percent increased risk). Instrument-assisted vaginal deliveries brought a 14 percent increased asthma risk during this 10-year period.
Overall, these findings confirm a moderate association between C-section delivery and later development of asthma, Tollanes and colleagues say. However, the reasons behind this association must still be determined.
According to Tollanes, there are two main theories about why C-sections could cause asthma. "The first is that babies who are born by Cesarean section are not exposed to their mothers' bacteria during birth, which is detrimental for development of the immune system," Tollanes explains in a written statement.
"The other is that babies born by Cesarean section have more breathing problems after birth because they are less exposed to stress hormones and compression of the chest, since these mechanisms contribute to emptying the lungs of amniotic fluid. Maybe this can negatively affect lung function in the long term," Tollanes said.
"The fact that emergency Cesarean section apparently has a stronger effect on the risk of asthma than planned Cesarean section cannot easily be explained by any of these theories," Tollanes admits. "It is possible that there are other common causes that can induce the need for Caesarean section and the development of asthma."