As strange as it sounds, something as simple as a urinary tract infection (UTI) may increase the risk of future depression and autism in an unborn child. This is according to a new study, published in JAMA Psychiatry, involving just shy of 1.8 million children born in Sweden between 1973 and 2014.

It isn't the first to link mental health (depression) and neurodevelopmental conditions (autism spectrum disorder, or ASD) to conditions in the womb. In the past, scientists have identified correlations between birth season (and later, maternal influenza infection) and schizophrenia diagnosis – though further studies have produced something of a mixed bag of results. Previous research has also discovered links between specific infections (cytomegalovirus and herpes, for instance) and certain psychiatric disorders.

However, the effect of a more general infection on the unborn brain is (in the words of the study's authors) an "understudied" phenomenon. And so, to address the issue, researchers at the University of Gothenburg, Sweden, tracked the health of hundreds of thousands of Swedes up until 2014 using the country's inpatient record. By then, the eldest participants were 41 years old.

Negating the effects of delivery year, maternal age, and maternal health and lifestyle factors (like smoking and diabetes), the researchers found a 79 percent increase in ASD risk and a 24 percent increase in depression risk in those individuals whose mothers had received hospital treatment for an infection during pregnancy. What's more, it did not matter if this infection was severe like sepsis, influenza, pneumonia, or meningitis, or mild like a UTI.

However, there was no associated risk of bipolar disorder or psychosis (for example, schizophrenia) – two health conditions that were also monitored during the research. 

So, what is going on?

While the study reveals a significant correlation, it doesn't explain why it exists. Though the researchers do point to a study (in mice) that found infections and inflammation in the mother affected placental serotonin production and serotonergic neuron development in the fetus. They suspect certain inflammatory proteins can change gene expression in fetal brain cells.

"Our results cannot exclude the possibility of increased risk for psychopathologic conditions as a result of a dual 'hit': an inflammatory fetal brain injury on a background of genetic susceptibility," explained the study's authors.

Their advice for soon-to-be mothers?

"The results indicate that safeguarding against and preventing infection during pregnancy as far as possible by, for instance, following flu vaccination recommendations, may be called for," lead author Verena Sengpiel, an associate professor of obstetrics and gynecology at Sahlgrenska Academy, University of Gothenburg, said in a statement.