Respiratory issues (not asthma specifically) & csects are linked statistically BECAUSE of the large number of preterm births that happen via csects (emergency basis, or planned multiples) and IMMATURE lungs.
That's the vital point.
Sort of like saying that toothpaste causes red hair, because moms of redheads use toothpaste. Or that sick people take pills, if I don't take pills I songs get sick.
Reverse logic, or info missing parts (like preemies, or what the stats are for asthma from vaginal births)... Doesn't paint a complete picture. But it sure sounds good. (Hence also the autism scare, that black people can't swim, that ... Pick any kind of reverse or skewed logic).
A STATISTICAL CORRELATION is completely and totally different from CAUSAL EFFECT. Or everyone would have red hair.
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(My son has asthma and other respiratory issues. Vaginal birth. BUT even though he's 10 new pulmonologists always want to know about his birth. WHY? Because if he had premature lungs, was on a ventilator, on certain meds... Those things predict future respiratory problems. Which is pretty 'duh'. Born with massive pulmonary problems, has breathing problems later. BUT also not always true. Lots and lots of preemies or NICU full term babies go on to be 100% healthy. He was 100% healthy at birth, and developed yhese problems later. It just helps paint a full picture.))