Can the Placenta Transmit Cancer?

It has been discovered that there are indeed circumstances where a mother's cancer cells cross the placental barrier and are transmitted to the baby.

It is an extremely rare occurrence with only 17 probable cases having been reported worldwide.

Researchers are not yet sure how the cancer cells are able to bypass detection from the baby's immune system. There is a theory that as the cells are transmitted via the placenta, a mutation occurs causing the baby's immune system to not view the cells as invaders or foreign and therefore the immune system does not launch an attack.

http://www.guardian.co.uk/science/2009/oct/12/cancer-passed-from-mother-foetus