There is good evidence that if a woman is depressed, anxious, or stressed while she is pregnant, then there is an increased risk that her child will have emotional, behavioral, or cognitive problems. Her own biology must cause these effects, but it is not known how. One important line of research suggests that the function of the placenta changes in response to maternal mood in ways that may allow more of the stress hormone cortisol and the neurotransmitter serotonin to pass through to the developing fetus and affect brain development. A better understanding of these mechanisms should eventually help researchers devise interventions to improve child development and health.