IF YOU think comparing a fetus to a parasite is unkind, think again. The puzzle as to why a pregnant woman's immune system doesn't attack the fetus and placenta - both of which contain genetic material from the father - may finally have been cracked.
It seems the placenta produces hitherto unknown hormones containing the same molecule some parasitic worms use to avoid detection by the immune system. As well as helping the fetus avoid immune attack, the hormones may also summon extra blood and nutrients to the aid of an undernourished fetus.
The discovery of this possible chemical control system could herald new ways to prevent recurrent miscarriages and pre-eclampsia, a condition which can lead to convulsions, coma and death in pregnant women. It also sheds some light on why rheumatoid arthritis, an autoimmune disease, eases when women become pregnant.