For some women, pregnancy is a time of stress: They worry about the pregnancy, fret about birthing complications and agonize about the health of the baby. But all this anxiety apparently can affect the child in years to come.

A study first published last year in the journal Child Development asserted that women who are stressed during pregnancy are at greater risk of having children who develop anxiety or behavior problems.

Researchers from Catholic University of Leuven, Belgium, looked at data on 71 mothers and their first-born children. They evaluated the women while they were pregnant and again when the children were 8 or 9 years old. During the pregnancies, the women kept diaries on their anxiety levels; when the children were older, evaluations included input from a teacher and an impartial observer as well as from the mothers.

They found that "anxiety during pregnancy explained 22 percent of the differences children show in hyperactivity symptoms, 15 percent of the differences in 'acting out' and aggression problems, and 9 percent of the differences in anxiety," lead author Bea Van den Bergh, a Catholic University psychologist, told HealthDay .

Worry during the second trimester, or between 19 and 22 weeks of pregnancy, was particularly significant for anxiety risk, she said.

Van den Bergh said explanations for these types of problems can be attributed to "fetal programming hypothesis," which may affect certain biological systems and predispose the unborn child to certain diseases and emotional disorders.

With most pregnancies comes stress, she conceded, but she urged women to do what they could to stay relaxed.

"Try to understand that when you are stressed or anxious the body reacts and that [stress] hormones go to the fetus," she said.

Dr. Diana Dell, an assistant professor of obstetrics and gynecology, psychiatry and behavioral science at Duke University , agrees.

"Women are under enormous pressure to make perfect babies," she told HealthDay . "Even if her obstetrician asks if she's feeling stressed, a woman may not own up to feelings of anxiety or depression. What this study is saying is, moms who are under constant stress in the second trimester, those children have increased rates of some developmental disorders in childhood. When a mother is anxious or depressed during pregnancy, that's not a neutral impact."