Women who take certain heartburn medicines during pregnancy may increase the risk of having a child with asthma, according to new research.

Researchers led by the universities of Edinburgh and Tampere in Finland analyzed eight existing studies that included more than 1.3 million children. The team, which published their findings in the Journal of Allergy and Clinical Immunology, discovered that mothers who had been prescribed acid-blocking medications had children who were one third more likely to visit a doctor for symptoms of asthma.

The two main types of acid-blocking drugs had different levels of association. The authors found that H2 blockers — such as Pepcid (famotidine), Zantac (ranitidine) and Tagamet (cimetidine) — were associated with a 46 percent increased risk for childhood asthma. PPIs — such as Prevacid (lansoprazole), Nexium (esomeprazole) and Prilosec (omeprazole) — had a 30 percent increase in risk.

Heartburn occurs when stomach acid passes from the stomach back into the esophagus and can cause pain and discomfort. Acid indigestion is common in pregnant women because of changes in hormones and pressure from the growing womb.

It is unclear why acid-blocking drugs commonly used to treat heartburn is linked with childhood asthma. Scientists had previously thought that the medicines may impact the immune system, but studies have not made a link. Some animal studies suggest that the drugs may interfere with digestion and allow undigested food allergens to pass through to the fetus, according to the New York Times.

Despite an association between heartburn medications and childhood asthma, the researchers acknowledge that a causal connection cannot be proven.

“Our study reports an association between the onset of asthma in children and their mothers’ use of acid-suppressing medication during pregnancy,” said Dr. Aziz Sheikh of the Asthma UK Centre for Applied Research at the University of Edinburgh. “It is important to stress that this association does not prove that the medicines caused asthma in these children and further research is needed to better understand this link.”

Since the studies did not account for a bevy of other factors, more research is needed to determine whether the medicines affect the health of fetuses.

The authors of the study said that mothers should not change treatments based on their findings.

“It is important to stress that this research is at a very early stage and expectant mums should continue to take any medication they need under the guidance of their doctor or nurse,” Dr. Samantha Walker, Director of Policy and Research at Asthma UK, said. “The study points us towards something that needs further investigation which is why we need to see more research carried out into the causes of asthma, a condition that affects 5.4 million people in the UK alone.”

Heartburn Medications Linked to Other Issues

This latest research is just another in a string of studies linking heartburn medications with potentially serious side effects.

Earlier this month, a study published in the British Journal of Clinical Pharmacology found that PPIs and H2 blockers may put patients at an increased risk of developing two stomach infections. According to the study, people who took the heartburn medication were 3.7 times more likely to develop an infection that causes tens of thousands of deaths each year in the United States.

Preliminary research presented in November at the American Heart Association’s Scientific Sessions 2016 found that PPIs increased people’s overall stroke risk by 21 percent. One PPI, Protonix (pantoprazole), increased the risk of stroke by as much as 94 percent when taken at its highest doses.

The U.S. Food and Drug Administration (FDA) has taken aim at heartburn medications over the years. In 2011, the FDA issued a safety communication that PPIs increased the risk of fractures. The FDA also alerted patients taking PPIs that they may develop low levels of magnesium, putting them at risk for seizures, muscle spasms and irregular heartbeats.