A mother gives everything to care for her child.

Biologists have recently demonstrated the truth of that greeting-card adage at the molecular and cellular levels. It comes as no surprise that the prenatal environment supplies nourishment, hinders embryonic development in the presence of metabolites from smoke or drink--and can even influence which of a child's genes get turned on or stay silenced. Now French researchers have found that biochemicals from the mother can get involved directly in the development of the embryo and fetus. Besides blurring the physiological borderline between mother and embryo, the "maternal effect" may help reveal the causes of several diseases.