(HealthDay News) -- Over the past century doctors have suspected that cancers can spread from a pregnant woman to her fetus, but a genetically confirmed case reported this week from Japan suggests the phenomenon is real.
Mother-to-fetus transmission still likely rare, experts say, since the placenta acts as a barrier to cells from the mother, and the fetal immune system would reject and destroy cancer cells.
"Some 30 times reported in the past, mother and infant have appeared to share the same cancer, usually leukemia or melanoma," noted lead researcher Dr. Shuki Mizutani of the department of pediatrics at Tokyo Medical and Dental University.
"The suspicion has been that the cancer may have developed in the pregnant woman but then spread to the baby in the womb. There has however been no clear genetic evidence to support this interpretation, which was shown unambiguously by our study for the first time," Mizutani said.
The report is published in the Oct. 12 online edition of the Proceedings of the National Academy of Sciences.
For the report, Mizutani's team studied the case of a 28 year-old Japanese woman who developed leukemia after giving birth. At 11 months, her baby girl developed the same type of leukemia.
A genetic analysis found that both mother and daughter had an identical match for a mutated cancer gene called BCR-ABL1. However, this mutation had not been inherited in the child's DNA, which suggested that the cancer cells had developed independently in both the mother and her fetus.
Using genetic "fingerprinting," the researchers found that the cancer cells had developed in the mother and were transmitted to her baby in utero.
In addition, Mizutani's team found that these cancer cells had destroyed an area of the infant's DNA that is normally able to distinguish between the infant's and mother's cells.
Based on this finding, the researchers believe cancer cells from the mother crossed the placenta and succeeded in implanting themselves into the fetus, unrecognized by the developing immune system.
The finding isn't all that surprising, one expert said.
"People have believed that this has been the case for some time. This is really crossing the Ts dotting the Is and showing that that's really the case," said William H. Chambers, scientific program director at the American Cancer Society.
He stressed that this type of transmission remains a rare occurrence. "I don't think people are going to decide that there are a whole lot more of these cancers," he said. "It's pretty rare that someone is going to find out they have a disease like this during or after pregnancy."
Mizutani agreed, but said precautions can still be taken.
"Malignant tumors are estimated to develop in one case of 1,000 pregnancies," Mizutani said. "The frequency of developing leukemia is estimated to be 1 in 75,000 to 100,000 pregnancies. Thus, although it might be rare, babies born in pregnant women who develop cancer during pregnancy should be placed under careful observation."
For more information on leukemia, visit the American Cancer Society.http://www.cancer.org/docroot/CRI/CRI_2x.asp?sitearea=LRN&dt=62