Pregnancy is a complex and fascinating process. There is an organism growing inside your womb and, as if it were not enough, it only shares half of your genetic heritage. The immune system is programmed to reject anything that is non-self, yet it tolerates the foetus. Mammalian placenta is a sophisticated organ that not only nourish the foetus, but it also makes feto-maternal immune tolerance possible. Cancer cells share many similarities with placental cells and maybe use the same mechanisms to escape the immune system.