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They are more likely to develop acute lymphoblastic (ALL) later in life, scientists say.

Previous research has shown that increase the , allergies and .

The new study, published in , looked at data from nearly 2.5 million children born in Sweden during two periods, 1982 to 1989 and 1999 to 2015.

Of those, 15.5 percent were born by C-section. Of the entire group, 1,495 children later developed leukemia.

Using the , the research team was able to divide the C-sections into planned and emergency caesarean sections.

The children who later developed leukemia were overrepresented in the group born by planned C-section.

In particular, the risk of the most common form of childhood leukemia, acute lymphoblastic leukemia, or ALL, increased.

The risk of ALL was 21% higher in children born by planned C-section compared with children born naturally.

The risk of developing the most common form of ALL, B-cell acute lymphoblastic leukemia (B-ALL), was 29% higher in those born by planned C-section.

The increased risk remained even when the researchers adjusted for other relevant factors in mothers and their children.

The increased risk was more pronounced in boys than in girls and among younger children, according to the findings.

But the research team from the in Sweden emphasized that the risk remains low, regardless of the mode of delivery.

They said between 50 and 70 Swedish children per year are diagnosed with B-ALL.

The excess risk associated with planned C-sections corresponds to around one case of B-ALL per year, according to the researchers.

Study lead author said: “C-sections are an important and often life-saving part of obstetric care.

"We don't want mothers to feel anxious about medically indicated C-sections.

"But when this result is combined with other study results showing that the risk of later asthma, allergies or type 1 diabetes increases in children born by planned C-section, there is reason to discuss C-sections that are not medically indicated."

The researchers suggest that the reason planned and not emergency C-sections carry an increased risk of certain diseases, all of which are related to immunological factors, could be that the baby does not experience stress and is not exposed to vaginal bacteria.

Dr. Kampitsi added, "Fortunately, ALL is rare.

"This means that many C-deliveries are needed to obtain a statistically significant result, and it is difficult to obtain such a large sample in a Swedish registry study.

"However, the results are close to significant, are in line with what previous studies have shown, and remain when we adjust for other relevant factors, which still makes them relevant."