Newborn triplets in Mexico have all tested positive for the coronavirus in what experts are calling an “unprecedented” case, according to a report.

The triplets were born prematurely on June 17 and all tested positive for COVID-19 on that same day, the BBC reported. Two of the babies – a boy and a girl – are in stable condition at a hospital in San Luis Potosí state, while the second boy is being treated for a respiratory condition, the report said.

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It marks the first time worldwide that the infection was detected in multiple births, a spokesperson for the state's Health Safety Committee said. Now experts are investigating whether the disease was passed on through the mother’s placenta during pregnancy.

"It would be impossible for them to have been infected at the moment of birth,” said State Health Secretary Mónica Liliana Rangel Martínez, according to the report.

The parents, who authorities say may have been asymptomatic, are being tested for COVID-19. Mexico has seen 185,122 infections and 22,584 deaths from the disease, according to Johns Hopkins University.

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The Center for Disease Control and Prevention (CDC) issued guidelines last month which said babies who contract the virus are likely exposed to infectious respiratory droplets from their mothers, caregivers, visitors, or health care personnel with COVID-19 after birth.

It said “limited reports” have raised concerns over possible transmission during birth.

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“Efforts are underway to address the knowledge gap of transmission between mother and neonate during pregnancy, delivery and in the postpartum period, and recommendations will be updated as new information informing the risk-benefit of maternal-infant separation is available,” the CDC said.