New research, which has been published in the journal Science Advances, explains how giving birth through a cesarean delivery may lead to weight gain in the offspring.

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As more and more women opt for a cesarean delivery, we may need to develop new ways to restore babies' natural mix of bacteria.

The World Health Organization (WHO) recommend that the total number of cesarean deliveries does not exceed 15 percent of live births.

Most cesarean deliveries are required to save the life of either the mother or the baby. However, they may be recommended for other medical reasons, and mothers sometimes opt for an elective surgery.

All in all, recent years have seen an increase of 50 percent in cesarean surgeries worldwide. In the United States, it is estimated that almost a third (or 32.5 percent) of all live births are through a cesarean delivery.

Researchers from the New York University (NYU) School of Medicine in New York City - led by Dr. Maria Dominguez-Bello, a microbiologist at NYU's Sackler Institute - set out to investigate the link between having a cesarean delivery and the weight of the offspring.

The research builds on previous studies that have demonstrated a link between having a cesarean delivery and the risk of obesity in the children. One such recent study reports that children born via cesarean are 15 percent more likely to become obese, and the increased risk is independent of the mother's weight.

But what explains this risk? One theory is that a cesarean delivery interferes with the natural transmission of maternal microbes through the mother's vaginal flora.

Such an early acquaintance with bacteria informs the baby's immune and metabolic systems, preparing them for a healthy adulthood.

While most studies have been observational and pointed merely to an association between weight gain and the delivery method, this study points to a causal mechanism that may explain this association.

The new research was conducted in mice, and, as Dr. Dominguez-Bello notes, it is "the first to demonstrate a causal relationship between [cesarean delivery] and increased body weight in mammals."

Cesarean-born females 70 percent heavier

Dr. Dominguez-Bello and her team compared 34 mice that had been delivered via cesarean with 35 control mice born through their mother's birth canal.

In an attempt to examine the effect of the delivery method on the microbiome of the mice, the researchers extracted and analyzed the DNA of the bacteria found in the offspring.

The researchers were also able to observe the changes in bacteria throughout development into adulthood.

They found that the microbiomes of the cesarean-born mice differed significantly from those of the mice born "naturally." The latter had certain bacteria - such as Bacteroides, Clostridiales, and Ruminococaceae - that have previously been associated with a leaner body type.

Additionally, the bacterial composition of naturally born mice matured normally into adulthood, whereas that of those delivered through cesarean did not.

The authors report that in the cesarean-born mice, the structure of the microbiome matured too quickly at first but then stagnated. Again, these differences in delivery method were most pronounced in females.

Furthermore, the study revealed significant differences in body weight depending on the delivery method, with the authors explaining that "mice gained 33 [percent] more weight at age 15 weeks if they were born by [cesarean delivery]," and for females it was 70 percent.

"Further research is needed to determine whether the dominance of certain bacterial groups can protect against obesity [...] Our results support the hypothesis that acquiring maternal vaginal microbes is needed for normal immune and metabolic development."

Dr. Maria Dominguez-Bello

"The question of whether a baby's founding microbiome affects its future obesity risk becomes more urgent as [cesareans] are increasingly used by choice in many parts of the world," she adds.

The authors note that if human studies confirm that that is the case, swabbing infants with the mother's birth fluid may replenish some of the lost bacteria. 

Speaking to Medical News Today about the findings, Dr. Dominguez-Bello said, "The results open possibilities to restoration with microbiota that is protective to the increased risk for the relevant disease."

"We still need to find," she said, "what human microbes are anti-obesogenic, and which are responsible of the increased risk, and why." 

"Labor and delivery [have been] medicalized excessively, and [need] to be re-humanized," Dr. Dominguez-Bello adds. "We need to use our technology respecting our biology."