Fresh data from Sweden is raising difficult questions for doctors and expectant parents, pointing to a small but measurable increase in childhood leukaemia among children born by planned caesarean section.
What the new Swedish study actually found
Researchers at the Karolinska Institutet in Stockholm analysed health records for more than 2.4 million children born in Sweden, following them for cancer diagnoses later in childhood. The work, published in the International Journal of Cancer in July 2025, focused on acute lymphoblastic leukaemia (ALL), the most common type of childhood cancer.
Planned caesarean sections carried a higher risk of acute lymphoblastic leukaemia than vaginal births or emergency caesareans, according to the study.
Sweden provided an unusually detailed set of data. The national birth registry allowed scientists to separate:
- Planned caesareans: scheduled before labour starts
- Emergency caesareans: decided during labour due to complications
- Vaginal births
That distinction turned out to be crucial. The increased risk was linked only to planned caesareans, not to operations carried out in an emergency once labour was already under way.
After adjusting for a long list of potential confounders — including birthweight, gestational age, birth order, parents’ education, and maternal smoking — the association between planned caesarean and a higher risk of ALL-B (the most frequent ALL subtype in children) persisted.
For ALL-B, the risk increase was estimated at around 29% for children born by planned caesarean, compared with those born vaginally.
On an individual level, that still translates into a very low absolute risk. In Sweden, only around 50 to 70 children develop ALL each year nationwide. The researchers calculated that planned caesareans might add roughly one extra case of ALL per 100,000 births annually.
Why a small risk still matters at population level
For any one baby, the chance of developing ALL remains tiny, regardless of delivery mode. Yet when a procedure becomes common across a whole country, even a modest shift can matter for public health planning.
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In Sweden, nearly one in six babies is now born by caesarean, with a substantial share of those operations booked in advance, not performed for a sudden emergency. When millions of births are involved, the numbers begin to stand out statistically.
The team also checked whether planned caesarean birth appeared linked to other childhood cancers, such as brain tumours or lymphomas. No similar signal emerged there, which strengthens the idea that something specific may be happening with ALL and early immune development.
How a baby is born and the developing immune system
The microbiome: first contact with bacteria
One leading hypothesis centres on the microbiome — the vast community of bacteria and other micro-organisms that begin colonising the body from the moment a baby arrives.
During a vaginal birth, and even during many emergency caesareans once labour has started, newborns are heavily exposed to their mother’s vaginal and gut bacteria. These microbes seem to “train” the immature immune system, teaching it to distinguish between harmless and harmful signals.
In a planned caesarean, that first contact looks different. The baby is lifted directly from the uterus in an operating theatre, often with limited exposure to maternal bacteria. Instead, early colonisation may be dominated by microbes picked up from the hospital environment, staff skin, and the air.
Researchers suspect that this altered microbial handshake at birth may subtly redirect the way the immune system develops over months and years.
Some scientists think that if immune regulation is skewed at this very early stage, it could leave a small window for abnormal white blood cells to gain a foothold, eventually leading to conditions like ALL.
The missing stress of labour
Another line of thinking focuses on stress hormones. Labour is physically demanding for babies as well as mothers. The contractions and passage through the birth canal create a short, intense stress that triggers the release of hormones such as cortisol and adrenaline in the newborn.
Far from being purely negative, this surge appears to help prepare the lungs, metabolism and immune system for life outside the womb. In a scheduled caesarean completed before labour starts, much of that hormonal cascade is blunted or absent.
Researchers working on the Swedish data suggest that missing these signals could tweak the timing and pattern of immune cell development. Those shifts might only increase risk by a fraction, yet across millions of births, the effect becomes visible in large registries.
Not all caesareans are equal
A key message from the Swedish team is that caesareans performed for clear medical reasons remain vital. When the baby is in distress, when labour stalls, or when the mother faces complications such as severe pre-eclampsia, surgery can save lives.
The concern raised by this study targets “caesareans of convenience” — operations scheduled without a strong clinical indication.
To help make sense of the risk, it helps to compare delivery options:
| Type of birth | Timing | Key features |
|---|---|---|
| Vaginal birth | After labour begins | Full exposure to birth canal microbiota and normal labour stress |
| Emergency caesarean | During labour | Some exposure to maternal bacteria and labour-related hormones |
| Planned caesarean | Before labour starts | Limited contact with maternal microbiota; reduced labour stress |
The Swedish findings suggest that the third category, planned caesarean before labour, is where the leukaemia signal appears. Emergency caesareans, although still surgical, did not show the same pattern in this study.
How this fits with previous research
This is not the first time scientists have linked delivery mode with long-term conditions. Earlier work in various countries has reported higher rates of asthma, allergies and type 1 diabetes among children born by caesarean, particularly when the operation is scheduled ahead of labour.
The new Swedish cohort adds cancer to that list of possible concerns. While some sub-analyses in the paper did not meet strict thresholds for statistical certainty, the overall pattern aligns with these previous observations, which researchers see as a meaningful convergence.
What this means for parents and clinicians
For pregnant people who medically need a caesarean, the message is reassuring: the absolute risk of childhood leukaemia remains very low, and the immediate safety benefits of surgery can far outweigh distant and uncertain harms.
The debate intensifies when requests for caesarean are driven mainly by anxiety, convenience or pressure to schedule the exact birth date. In those cases, doctors may now feel on firmer ground suggesting a vaginal birth, or at least taking more time to discuss long‑term trade‑offs.
The study does not argue against caesareans as a life‑saving procedure; it questions routine scheduling when medical need is unclear.
Expectant parents facing this decision might want to ask their care team pointed questions, such as:
- What specific medical reasons support a caesarean in my case?
- Could we safely wait for labour to begin before considering surgery?
- How will my baby be supported if we aim for vaginal birth and plans need to change?
Terms and concepts that often cause confusion
The discussion around this research uses some specialised terms:
- Acute lymphoblastic leukaemia (ALL): a cancer of the blood and bone marrow, where immature white blood cells (lymphoblasts) grow uncontrollably.
- ALL-B: the most common subtype of ALL in children, involving B‑cell lymphoblasts.
- Microbiome: the complex community of bacteria, viruses and fungi that live on and inside the body, particularly in the gut.
- Absolute risk: the actual chance that something happens (for example, 1 in 100,000).
- Relative risk: a comparison between two groups (for example, 29% higher than another group).
Understanding the gap between relative and absolute risk helps put headlines into perspective. A “29% increase” sounds dramatic at first glance, but if the starting risk is extremely low, the added danger for any single child stays low as well.
What could happen next in maternity care
If similar results appear in other countries’ registries, health authorities may revisit guidelines for elective caesareans. Hospitals could, for example, tighten criteria for scheduling operations before labour, or encourage shared decision‑making that fully addresses long‑term outcomes alongside short‑term comfort.
Some maternity units are already experimenting with practices such as “vaginal seeding”, where gauze swabs transfer maternal vaginal bacteria to babies born by caesarean. This idea remains controversial and is not widely recommended yet, but it shows how seriously clinicians are starting to take the early microbiome.
For now, the Swedish data adds one more argument for careful, case‑by‑case decisions. Surgery will remain a crucial tool in obstetrics. The new concern is not the operation itself, but a drift towards using it when nature was probably coping just fine.
Originally posted 2026-02-19 06:29:52.