Scientists have discovered a new hormone emitted by the fetus into the placenta called adrenomedullin that they believe is crucial in the prevention of a woman getting pre-eclampsia during pregnancy. Adrenomedullin is responsible for assisting a woman's blood vessels to dilate, accommodating for the extra blood flow. Scientists are hoping adrenomedullin can lead to discoveries about which women may be predisposed to pre-eclampsia.
Read more about it: http://www.futurity.org/health-medicine/babys-cells-emit-hormone-to-prevent-preeclampsia/
It should also be noted that pre-eclampsia is believed to have many causes and factors, one of those being nutrition. It's important to have sufficient nutrition during pregnancy, and this can also help prevent pre-eclampsia.