Dr. Andy

Reflections on medicine and biology among other things

Friday, March 04, 2005

Do C-sections cause Asthma?

Does route of deliver (C-section versus vaginal) affect the risk of asthma? A recent study suggests that it increases the risk, but only among premature not-full term infants. Previous studies have found mixed results.

This studies illustrates the vagaries of statistics. In general studies report the increase in risk and then a 95% (or 99%) confidence interval, which gives you a range into which the "true" difference is likely to fall. So if this confidence interval doesn't overlap 1 (or no difference between the 2 groups, these are usually expressed as a ration) you are supposed to believe that the difference is real

The overall risk of asthma was 20% higher in all births (odds ratio 1.2), but the 95% confidence interval is 1.04 to 1.39, which is barely significant (BTW I looked at original study which is not freely available online).

When they broke out premature vs. term delivery, the found a big increase in the premature group (OR 1.9, range 1.09-3.02) but a (barely insignificant difference in term kids (OR 1.15, but 95% confidence interval 0.97 to 1.34, meaning the difference might have occurred by chance).
A little bigger study might have reached significance for term kids.

What is the link? Well one possibility is that mom's with asthma are both
1. more likely to have c-sections (some data supports this)
2. more likely to have kids with asthma (a ton of data supports this)

They had data about maternal asthma in only a few of the mothers.

Another possibility is that mode of delivery alters which bacteria colonize the baby's gut. This has been shown to happen and differenes in intestinal flora to influence allergies and asthma

So overall I'd say there is a significantly increased risk in premature babies born by C-section, maybe a small increase in full term, but whether the increase is casual remains to be seen.
I think vaginal delivery is generally preferred, but I doubt C-section is playing a significant role in the asthma epidemic.

Full Disclosure: both Son of Dr. Andy and Daughter of Dr. Andy were born by C-section.


At 12:36 PM, Blogger doulicia said...

Thanks for the intro stats lesson. I really need to relearn some basic statistics to make any sense of all the studies out there.

At any rate, I thank you for taking the time to reflect on it.

At 9:20 AM, Blogger jon said...

Looking at asthma exercise info online today while my son coughs I came across this post. Does anyone know a good asthma exercise site to help?



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