Another day, another poorly done study that claims to show that C-sections are harmful.
According to The Sydney Morning Herald, Caesareans linked to slower start at school: research.
[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]The authors fail to control for the most important confounding variable, the risk that the infant sustained brain damage before birth.[/pullquote]
Children born via caesarean appear to lag behind their peers – who were delivered naturally – in school tests, new research has found.
While the gap in test scores is small, University of Melbourne researchers say it’s equivalent to a grade 3 student missing up to 35 days of school.
And they suspect different bacteria in the guts of babies delivered by caesarean could be behind the delay.
In fact, the study showed nothing of the kind. Indeed, it showed nothing at all for three important reasons:
- The authors didn’t correct for the single most critical confounding variable, oxygen deprivation.
- The effect size was tiny.
- The claim that the microbiome of C-section babies differs from that of vaginally born babies has been debunked.
As a result, the paper does not support the claims that the authors made.
The paper is The relation between cesarean birth and child cognitive development published in the journal Scientific Reports. It was written by two economists and a basic scientist who investigates neurophysiology.
Across several measures, we find that cesarean-born children perform significantly below vaginally-born children, by up to a tenth of a standard deviation in national numeracy test scores at age 8–9. Estimates from a low-risk sub-sample and lower-bound analysis suggest that the relation is not spuriously related to unobserved confounding. Lower rates of breastfeeding and adverse child and maternal health outcomes that are associated with cesarean birth are found to explain less than a third of the cognitive gap, which points to the importance of other mechanisms such as disturbed gut microbiota. The findings underline the need for a precautionary approach in responding to requests for a planned cesarean when there are no apparent elevated risks from vaginal birth.
Where did the authors go wrong?
In any study, it is critically important to ensure that the two groups under study do not differ in any meaningful way from each other. For example, many breastfeeding studies produce spurious results because children who are breastfed differ economically from those who are not. The purported health benefits of breastfeeding are therefore likely to be benefits of being wealthy (which we know has a significant impact on health), not of being breastfed.
In this study, the single most important confounding factor that must be taken into account is brain health at birth. That’s why most studies that compare C-section babies to those born vaginally take care to limit the C-section group to non-emergencies. Emergency C-sections are typically performed for fetal distress presumed to be caused by oxygen deprivation. Therefore, the C-section group is almost guaranteed to contain some babies who have been harmed by lack of oxygen. Restricting the C-section group to elective surgeries limits that possibility.
The authors in this study corrected for nearly two dozen variables:
The analysis includes over 20 confounders grouped into two main categories (Table 1): those related to perinatal risk factors and those related to the socio-economic advantage associated with cesarean-born children in Australia. Perinatal risk factors include the taking of medication during pregnancy for blood pressure or diabetes (proxies for pre-eclampsia and gestational diabetes respectively), the taking of antibiotic medication (a proxy for bacterial infection, which may also affect the development of the infant’s gut microbiome); a dummy variable for low birth weight (coded 1 if less than 2.5 kg; 0 otherwise); weeks of gestation; maternal age at birth; dummy variable for multiple infant pregnancy; length and head circumference of baby (z-scores); dummy variable for whether the baby was conceived using IVF treatment and a gender dummy. We include taking antibiotic medication as a control because it has been associated with changes to the infant’s gut microbiome and possibly the risk of cesarean birth, which means failure to control for it will lead to bias due to unobserved confounding.
Yet they fail to control for the most important confounding variable of all, the risk that the infant sustained brain damage before birth. Since the authors can’t be sure that the babies in each group were cognitively equivalent at the outset, they can’t conclude that observed cognitive differences were due to C-sections.
A second factor undermining the authors’ claims is that the difference in cognitive ability was extremely small. The effect size was less than 0.1 standard deviation.
What is effect size?
The article It’s the Effect Size, Stupid; What effect size is and why it is important explains the difference between statistical significance and effect size:
‘Effect size’ is simply a way of quantifying the size of the difference between two groups. It is easy to calculate, readily understood and can be applied to any measured outcome in Education or Social Science. It is particularly valuable for quantifying the effectiveness of a particular intervention, relative to some comparison. It allows us to move beyond the simplistic, ‘Does it work or not?’ to the far more sophisticated, ‘How well does it work in a range of contexts?’ Moreover, by placing the emphasis on the most important aspect of an intervention – the size of the effect – rather than its statistical significance (which conflates effect size and sample size), it promotes a more scientific approach to the accumulation of knowledge. For these reasons, effect size is an important tool in reporting and interpreting effectiveness.
In this study, the effect size was less than 0.1. How do we interpret that?
Another way to interpret effect sizes is to compare them to the effect sizes of differences that are familiar. For example, describes an effect size of 0.2 as ‘small’ and gives to illustrate it the example that the difference between the heights of 15 year old and 16 year old girls in the US corresponds to an effect of this size. An effect size of 0.5 is described as ‘medium’ and is ‘large enough to be visible to the naked eye’. A 0.5 effect size corresponds to the difference between the heights of 14 year old and 18 year old girls. Cohen describes an effect size of 0.8 as ‘grossly perceptible and therefore large’ and equates it to the difference between the heights of 13 year old and 18 year old girls. As a further example he states that the difference in IQ between holders of the Ph.D. degree and ‘typical college freshmen’ is comparable to an effect size of 0.8.
So an effect size of less than 0.1 is tiny and therefore, not particularly meaningful.
Finally, the authors offer an explanation for the purported difference between C-section babies and vaginally born babies that has already been debunked.
According to the authors:
The direct association may occur through alterations to the infant’s gut microbiota. Unlike vaginally-born children whose gut is seeded by passing through the birth canal, the gut of cesarean-born children is seeded through contact with the mother’s skin and hospital surfaces. The result is long-term compositional differences in gut microbiota by mode of birth with differences observed up until age seven…
There is absolutely nothing in this study that gives credence to this explanation, and the authors acknowledge that this theory has yet to be proven in any context:
Although causal impacts on child development are yet to be proven, altered signaling from disturbed gut microbiota is thought to be a possible driver of higher rates of cognitive disorders, especially autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD), among cesarean-born children …
In addition, a larger, more recent study has debunked the theory that the infant microbiome differs appreciably between C-section and vaginally born babies. The newer study concluded:
[T]here was no discernable effect of the cesarean mode of delivery on the early microbiota beyond the immediate neonatal period (and never inclusive of that in the meconium or stool) …
The bottom line then is that there is NO EVIDENCE that C-sections lead to cognitive delays.
Any study that claims to show that C-section babies have cognitive delays must correct for hypoxic birth injuries, have a moderate to large effect size and be based on a plausible biological mechanism.
This study strikes out on all three counts.
