The case was studied by cell biologist Mel Greaves of the Institute of Cancer Research in Sutton, Surrey, United Kingdom, who studies transmissible cancers. Greaves and her colleagues discovered several pieces of evidence backing the theory that cancer was transmitted from mother to fetus.

They found incipient cancer cells in blood samples taken from the girl at birth. Further study showed that the baby’s tumor had an identical DNA signature to the mother’s disease. The tumor also had almost no genetic material from the father, which suggests that cancer developed in the mother and infected the baby, rather than vice versa.

The team also found out how cancer managed to slip though the placental barrier. The disease lacked a portion in its DNA, which produces surface markers that immune cells latch on to. Greaves described it as being “immunologically invisible”.

The study is not likely to change how pregnant women are screened or cared for. Such cases are extremely rare, with only a few dozen suspected over the last century and a half, but it gives medics a new insight into how cancer interacts with the immune system.

The Japanese girl studied by Greaves and colleagues was successfully treated and is alive and well.