It's not impossible for an infant to get cancer from their mother, but it doesn't happen very often.

Researchers estimate transmission of maternal cancer to offspring occurs perhaps only twice in every million births for mothers with cancer, with the diseased cells being delivered to the infant via the placenta in these exceedingly rare and unfortunate cases.

But that's not the only way a mother with cancer might unknowingly infect her child, scientists say.

In a new case study published by researchers in Japan, doctors report what appears to be a medical first: vaginal transmission of cancer cells from mothers with cervical cancer to infants at the moment of their birth.

"Mother-to-infant transmission of tumour in the birth canal during vaginal delivery is also theoretically possible," the researchers, led by paediatric oncologist Ayumu Arakawa from the National Cancer Centre Hospital in Tokyo, explain in their paper.

"If the mother has cervical cancer, the infant can be exposed to tumour cells in fluids in the birth canal and could aspirate tumour cells into the lungs."

This obscure and previously unknown vector – inhaling cancer in your very first breaths, ostensibly – appears to be responsible for two otherwise unlinked cases of paediatric lung cancer diagnosed in a pair of young male patients: a 23-month-old and a six-year-old.

In both cases, the patients presented to hospital with symptoms of their illness, the 23-month-old experiencing coughing, and the six-year-old experiencing chest pain.

In each instance, CT scans revealed what eventually turned out to be cancerous tumours in the boys' lungs, which were successfully treated in both cases – although not easily.

Both the young patients received repeated bouts of chemotherapy, but were only cleared of disease after having operations to remove the cancerous tissue: in the case of the 23-month-old, a cancerous lobe of lung tissue was taken out, while the six-year-old needed his entire left lung removed.

While it's impossible to know for sure how these boys developed the cancerous growths in their lungs, the researchers say they've identified the most probable cause.

In the case of the 23-month-old, his 35-year-old mother, who had not received a vaccination against human papillomavirus (HPV), received a diagnosis of cervical cancer just three months after the infant's birth, having tested negative seven months before. Eventually, the tumours spread to her lung, liver, and bones, and she died.

However, analysis of the tissue from her cervical tumour and her son's lung tumour revealed gene profile similarities in the form of certain mutations and alleles, and the lack of the Y chromosome – suggesting the boy's cancer had come directly from his mother.

The six-year-old's tumours bore the same genetic signatures of his own mother's cervical cancer – which was first observed via a tumour that was detected during her pregnancy, but considered, unfortunately incorrectly, to be stable. She later had her uterus removed, along with her fallopian tubes and ovaries, but passed away two years after the surgery from the disease.

Based on all the above – and that both sons' tumours also bore the hallmarks of the HPV genome – the researchers concluded the lung cancer in the children was caused by transmission of cervical tumours from the children's mothers.

"The peribronchial [relating to airways of the respiratory system] pattern of tumour growth in both children suggested that the tumours arose from mother-to-infant vaginal transmission through aspiration of tumour-contaminated vaginal fluids during birth," the team explains.

In most cases involving mother-to-foetus transmission of cancer, the placental delivery of cancerous cells tends to result in the spreading of cancers to the brain, bones, liver, and other tissues, including the lungs. Here, it was just the lungs – which offers a strong clue as to how the disease could have been delivered to the boys' own bodies.

"In our two patients, tumours were observed only in the lungs and were localised along the bronchi," the authors write.

"It is likely that maternal tumour cells were present in the amniotic fluid, secretions, or blood from the cervix and were aspirated by the infants during vaginal delivery."

While the chances of this route of cancerous infection would appear to be incredibly slim for any individual family, the researchers stress that prevention via HPV vaccination would further prevent this method of transmission from occurring – an especially important message in the context of Japan, which has a troubled history with HPV vaccinations.

Further, the researchers suggest caesarean section should be the recommended form of birth for mothers with a history of uterine cervical cancer.

Beyond those takeaways, these grim cases of an extremely rare medical phenomenon serve as a remarkable example of how much we have yet to learn about cancer – and the insidious routes via which the disease appears to spread itself.

"I found it fascinating, personally," Debbie Saslow, the senior director of HPV-related and women's cancers at the American Cancer Society, who wasn't involved with the study, told Health Day.

"I didn't know this was possible."

The findings are reported in The New England Journal of Medicine.