Baby poop is changing, and that could be bad news for children's health.

Anyone with a newborn knows that baby poop is important. Pediatricians often ask parents of a new baby to keep track of what’s in their diaper, to make sure they are eating properly and that everything’s working as it should.

A new report published Wednesday finds that baby poop can tell us more than just how one infant is doing.

The study found that the pH, a measure of how acidic or alkaline something is, has been steadily going up since the 1920s.

This matters because acidity can tell doctors about the baby’s microbiome — the balance of “good” bacteria that help digest food and protect us from disease.

The researchers think it may also help explain the rise of allergies and asthma in the modern era.

Bethany Henrick of the University of Nebraska and Evolve BioSystems Inc. looked at medical studies of baby poo going back to 1926, when researchers first started characterizing the bacteria in infant feces.

“A review of 14 clinical studies published between 1926 and 2017, representing more than 312 healthy breastfed infants, demonstrated a change in fecal pH from 5.0 to 6.5,” they wrote in their report, published in the journal mSphere.

A low pH indicates a fluid is more acidic, while anything with a pH of 7 or higher is considered more alkaline.

The change has accelerated since 1980, they added.

Studies indicate that pH can be a quick way to measure whether an infant’s digestive system has enough beneficial bacteria from a group called Bifidobacterium. When these bacteria break down milk, they produce acids and that acidity shows up in the baby’s waste.

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There’s one particular species of Bifidobacteria that indicates a healthy gut. It’s called B. infantis. Babies with plenty of B infantis produced dirty diapers with more acidity than babies lacking in this bacteria, the researchers said.

Everyone carries a microbiome — a population of microbes such as bacteria and yeast that affect all sorts of functions, from absorbing food to immune response.

What is a healthy microbiome?

Babies are born almost sterile and they get the germs that will live with and on them from their mothers, their first meals, and their early handling and environment.

Researchers are trying to figure out what the elements of an optimal microbiome are. People with imbalances may be more prone to obesity, for instance.

And the digestive tract is an important front in the body’s immune system — something that makes sense, since animals take in many germs through food. The very first bacteria that show up in the digestive tract may influence how it develops.

A 2015 study showed that gut bacteria may affect a baby’s risk of asthma.

“There is clear evidence that the infant gut microbiome has important long-term health implications, and perturbations of the microbiome composition may lead to chronic inflammation and immune-mediated diseases,” Henrick and her team wrote.

“Thus, the loss of Bifidobacterium and the profound change in the gut environment, as measured by fecal pH, present a compelling explanation for the increased incidence of allergic and autoimmune diseases observed in resource-rich nations."

Three main factors could be killing off these beneficial bacteria, Henrick’s team said.

1. Infant formula

Giving babies formula instead of breastmilk can affect which bacteria thrive in their bodies

2. Antibiotics

Moms are often given antibiotics to protect babies at birth and these antibiotics could kill off good bacteria.

3. C-section rates

Caesarean section births could deprive infants of a chance to collect good bacteria as they are born. In the US, rates have been declining since 2007, but one-third of births, or 1.2 million babies, were delivered by C-section in 2015, according to the Centers for Disease Control and Prevention.

The research team all work at Evolve BioSystems, which is developing commercial probiotics — products that people can eat or drink to improve their microbiomes.

So far, scientists don’t know for sure which good bacteria we all need more of. But studies such as this one can help guide them.

“This steady increase in the fecal pH of infants over the past several generations has largely gone unnoticed by the medical community, but looks to be an indication of a major disruption of the infant gut,” said David Kyle, chief scientific officer at Evolve BioSystems, Inc.

“This may be a significant contributor to the incidence of allergic and autoimmune disorders."