THURSDAY, May 3 (HealthDay News) -- Children born after a longer-than-normal pregnancy are at increased risk for behavioral and emotional problems, a new study suggests.

The study found that attention-deficit hyperactivity disorder (ADHD) is an especially common problem among children who were born post-term, defined as birth after a pregnancy of 42 weeks.

The study of more than 5,000 infants in the Netherlands found that about 7 percent were born post-term, while 4 percent were born pre-term (before 37 weeks of pregnancy). Children who were born post-term and pre-term both had an increased risk of behavioral and emotional problems when they were 18 and 36 months old.

The post-term children were more than twice as likely as normal-term children to have ADHD symptoms, according to the study in the International Journal of Epidemiology.

The link between post-term birth and emotional and behavioral problems in early childhood did not appear to be explained by factors such as mother's weight and height, ethnicity, family income, alcohol consumption or smoking, education level, or the mother's mental health during pregnancy.

Although the study found an association between lengthy pregnancies and ADHD and other problems in babies, it did not prove causality.

More research is needed determine a causal relationship, as well as whether the link between post-term birth and emotional and behavioral problems continues past 36 months of age, lead author Hanan El Marroun said in a journal news release.

More information

The U.S. National Institute of Mental Health has more about attention-deficit hyperactivity disorder.