Are C-Section Babies Smarter? What the IQ Studies Show

No. The best studies find that babies born by C-section do not grow up smarter, and the gaps that survive adjustment for family and birth factors are small and slightly negative. A Danish register study of 1,408,230 people, the largest with an intelligence-type test, found that men born by C-section scored 0.36 points lower than men born vaginally on the Danish conscription board's 78-item test, about 4% of a standard deviation, or roughly 0.6 IQ points if one standard deviation is 15 (our conversion, not the paper's). A Brazilian cohort tested with the WAIS-III at 18 to 19 found a 5.8-point IQ advantage for C-section births, but it shrank to 1.9 points, no longer statistically significant, once the researchers accounted for socioeconomic and perinatal factors. The Danish authors concluded that school and conscription performance was "comparable regardless of delivery mode".

How common are C-sections, and why does the question matter?

The question has a large audience because the operation, spelled cesarean in the United States and caesarean in the United Kingdom, is now routine. The CDC's National Center for Health Statistics reported on 9 April 2026 that the U.S. cesarean rate rose to 32.5% of births in 2025, the highest since 2013, and that the rate for low-risk cesarean deliveries rose to 26.9%. The World Health Organization said in 2021 that C-sections account for more than 1 in 5 births worldwide (21%), up from around 7% in 1990, and projected 29% of births by 2030. When a third of births in a country are surgical, even a small effect on children's development would matter, so researchers have looked. This guide covers what they measured, because many pages that answer "does C-section affect IQ" quote studies of autism, ADHD or school grades as if they were IQ studies.

What did the Danish study of 1.4 million people find?

Ladelund and colleagues (Acta Obstetricia et Gynecologica Scandinavica, 2023) followed every Danish child born alive between 1978 and 2000. After exclusions the cohort held 1,408,230 children. Intelligence was available for men only, from the Danish conscription board, which uses the Børge Priens Prøve: 78 items, scored as the number answered correctly, in use since 1956. The authors cite earlier work showing that it correlates 0.82 with the full-scale Wechsler Adult Intelligence Scale, so it is a reasonable stand-in for an IQ test, although its scores are not on the IQ scale.

• Men compared: 81,995 born by C-section and 511,240 born vaginally
• Unadjusted difference on the 78-item test: -0.34 points (95% interval -0.43 to -0.26)
• Adjusted difference: -0.36 points (95% interval -0.46 to -0.27), which the authors put at "about 4%" of the vaginal-delivery group's standard deviation
• Elective versus acute C-section: no significant difference in the adjusted models (the split exists only for births in 1995 and 1996)
• Adjusted for gestational age, birth weight, delivery complications, parity, mother's age, maternal and paternal education, marital status and more

Two things follow. The adjusted and unadjusted gaps are almost the same, so family background did not explain the difference in Denmark; the gap is small before and after. And it is small in absolute terms: a person's own score on repeat testing moves by more than 0.6 IQ points, because every IQ score carries a margin of error (the WISC-V Full Scale IQ has a standard error of measurement of 2.90 points, as our margin-of-error explainer shows). A Norwegian study of conscripts' military intelligence test, reported in the Danish paper's discussion, also found no significant association between mode of delivery and low scores; we did not read the original.

Why did C-section teenagers in Brazil score 5.8 IQ points higher?

Lima and colleagues (Cadernos de Saúde Pública, 2023) used the São Luís birth cohort, which began in 1997 and was tested in 2016 with the WAIS-III at ages 18 and 19. The average IQ was 101.4. In the crude comparison, adolescents born by C-section scored 5.8 points higher than those born vaginally (95% interval 3.8 to 7.7). In the multivariate analysis, adjusted for socioeconomic variables at birth and perinatal variables, the difference fell to 1.9 points with a p-value of 0.141, which is not statistically significant. The authors conclude that C-section is not associated with the IQ of adolescents in this sample, and that the differences are explained by socioeconomic and perinatal factors. The abstract we read does not give the sample size, so we do not print one.

The Brazilian result is the cleanest illustration of the problem with raw comparisons. Who has a C-section is not random. The Australian researchers behind another study note that cesarean birth "is associated with higher perinatal risk factors and a socio-economic advantage, especially higher maternal education", and the Swedish authors point out that "foetal distress and maternal anxiety" can be reasons for an operation, which would create a link that is not caused by the surgery. So a headline claiming that C-section babies are brighter, or duller, needs the adjusted number beside it.

What do school-test studies in Australia and Sweden show?

Two large studies looked at schooling rather than IQ. Both are observational, and both found small negative gaps.

• Australia (Polidano, Zhu and Bornstein, Scientific Reports, 2017): 3,666 children in the Longitudinal Study of Australian Children. C-section children scored lower by up to a tenth of a standard deviation on national numeracy tests at ages 8 to 9, with smaller gaps in reading, writing and grammar. These are school and vocabulary tests, not IQ tests, and the study has no sibling comparison. In a subsample of low-risk births the gap was larger, and the authors say the mediators they tested, including breastfeeding and obesity, leave at least 70% of the relation unexplained
• Sweden (Curran and colleagues, BMC Pregnancy and Childbirth, 2017): 1,489,925 births in 1982 to 1995 and grades recorded at the end of compulsory school, at age 16, on a 0 to 320 scale. The adjusted odds of a poor result (below 160, a "pass") were 1.06 for elective C-section and 1.12 for emergency C-section against unassisted vaginal birth. The authors describe the effect as "quite small" and urge caution
• China (Xu and colleagues, Psychiatry Investigation, 2023): 10,568 questionnaires, with the China-Wechsler scale and Raven's matrices. Only children born by C-section before labor had a lower Performance IQ; verbal, full-scale and fluid intelligence showed no significant differences. We could not extract the point differences from the figures

If you convert the Australian gap to IQ-style points (a tenth of a standard deviation is 1.5 points at a spread of 15), it is about 1.4 points on a numeracy test. That is a rough conversion of a school test, not an IQ measurement, and it sits in the same range as the Danish and adjusted Brazilian figures: smaller than the margin of error of one IQ score.

Does C-section raise the risk of autism, ADHD or intellectual disability?

Many pages mix these questions up. A 2019 meta-analysis in JAMA Network Open (Zhang and colleagues; 61 studies, 20,607,935 deliveries) found that C-section was associated with autism spectrum disorder (odds ratio 1.33, interval 1.25 to 1.41) and ADHD (1.17, interval 1.07 to 1.26). For intellectual disability the odds ratio was 1.83, but the interval ran from 0.90 to 3.70, so it includes no effect. The estimates were comparable for emergency and elective operations. A January 2026 study in the same journal of 504,380 British Columbia children found comparable neurodevelopmental outcomes for second-stage cesarean and operative vaginal delivery, except for vacuum and sequential-instrument deliveries. These outcomes are diagnoses, not scores, so they say little about the average IQ of C-section children.

Is it the operation, or something else? Why sibling studies matter

The standard way to separate cause from background is to compare siblings born to the same mother by different routes. A 2023 meta-analysis in BMC Medicine pooled 18 studies and 21,854,828 people on outcomes that included mental and behavioral disorders, metabolic disease and asthma. The pooled odds ratio was 1.14 (interval 1.11 to 1.17) in whole-cohort studies and 1.08 (1.02 to 1.14) in sibling comparisons, which suggests that part of the whole-cohort association comes from family background. The authors argue that sibling designs may overcorrect, because siblings differ in the mother's age at birth. We did not find any sibling-comparison study that uses an IQ test as the outcome. That gap in the evidence is why "contested" is the honest label here.

Could the microbiome explain a difference? What the mechanisms are

Each proposed mechanism is a hypothesis. The Australian authors write that "causal impacts on child development are yet to be proven" and that altered signaling from disturbed gut microbiota "is thought to be a possible driver"; the Swedish authors list early-term birth, altered microbiota and stress response as possibilities. The nearest human test is a 2026 pilot randomized trial in Acta Obstetricia et Gynecologica Scandinavica of 210 mother-baby pairs (70 per arm) in which C-section babies received vaginal microbiota. The difference in the ASQ-3 total developmental score was 8.90 points (interval -2.9 to 20.7), not significant, and the authors urge caution. That is a developmental screening questionnaire, not an IQ test. A University of Bergen press page from January 2024 was headlined "Being born by Caesarian section may affect your intelligence"; it describes a gene-by-delivery analysis in 2,421 children and quotes its own researcher calling the study "relatively underpowered".

What should parents and readers take from this?

Mode of delivery is a medical decision, and nothing in the IQ literature gives a reason to choose or avoid a C-section. The adjusted gaps found for IQ-style tests are under about two points either way, and the size of the effects of other early-life factors dwarfs them: see our blog guides to premature birth and IQ, breastfeeding and IQ and prenatal smoking and IQ, and the news pieces on twins and singletons, birth order and screen time in children. For how much of a person's score is shared with family, see what twin studies show. Read any single-number claim in this area against the margin of error, and ask for the adjusted figure and the test.

The short version: the best IQ-type studies find a gap of well under one IQ point in Denmark, a Brazilian advantage that disappears after adjustment, and school-test gaps near a tenth of a standard deviation, none of them proven to be caused by the operation. If you want to see what a gap of a point or two means for one person, the IQ percentile calculator converts any score on the mean-100, standard-deviation-15 scale into a percentile, and our IQ Metrics test reports its score with a range for the same reason.

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