On occasion, the immune system is primed to attack the embryo because it contains genetic material from the father, which can be interpreted as foreign.

BBC: NEWS | Health | Test to boost IVF success rates

The researchers believe that sHLA-G appears to protect the developing embryo from attack by the mother's immune system.

BBC: NEWS | Health | Test to boost IVF success rates

Professor Alan Beer from Chicago Medical School, says that, in some cases, the problem may be an immune system which turns on the newly-implanted embryo and destroys it.

BBC: Immune treatment 'cuts IVF failures'

Could patients have themselves cloned in order to harvest cells from a genetically-identical embryo, gaining a donor and avoiding the major problem of transplant surgery: rejection of foreign tissue by the immune system?

CNN: GROWN TO ORDER