A Range of Possible Causes

What triggers new parents' scary thoughts? No one knows for sure, but here are some theories:

· It's possible that hormonal fluctuations in late pregnancy and the postpartum period set the stage by altering the levels of neurotransmitters, particularly serotonin, in the brain. "The evidence for this is a lot of people [with obsessive-compulsive disorder] have premenstrual exacerbation of obsessive symptoms," Nestadt says.

· Another theory suggests that the sheer stress of caring for a newborn, with all the adjustments this entails, may trigger disturbing thoughts. After all, the postpartum period is "a time when a new parent is immediately hit with a ton of responsibility -- suddenly, they have an intensely vulnerable being they're responsible for," Abramowitz notes. "Whenever a person is under stress, they tend to have more bad thoughts about what could go wrong. It's just part of how the brain works."

· An evolutionary theory proposes that intrusive thoughts involving aggressive acts against an infant may have survival value by causing parents to be extra-protective of the child.

· Lack of sleep makes people more vulnerable to stress, so the exhaustion that affects many new parents probably contributes, too. "A person's ability to decide whether to put stock in these thoughts depends on how exhausted they are that day," Dell says. "Fatigue is such a big factor. Exhaustion, even in a person who is normal, will amplify the intrusive symptoms."

-- Stacey Colino