Babies who spend longer than 42 weeks in the womb are more than twice as likely to develop behavioral and emotion problems like attention deficit hyperactivity disorder in early childhood compared to children born normal-term, according to researchers. 

Researchers from the new study of 5,145 Dutch babies born between 2001 and 2005 concluded that "post-term children have a considerably higher risk of clinically relevant problem behavior and are more than twice as likely as term born children to have clinical ADHD,” according to a statement released on Thursday.

Children born before 37 weeks are considered “pre-term,” “term” if they are born between 37 to 42 weeks or “post-term” if they are born after 42 weeks.

Experts say that the added risk for emotional and behavioral problems in early childhood in post-term babies did not appear to be influenced by factors like maternal weight and height, ethnicity, family income, alcohol consumption, smoking habits, education level or the mother’s mental health during pregnancy.

The findings show a U-shaped correlation between age at birth and behavioral and emotion problems in early childhood, indicating that both pre-term and post-term babies are at higher risk for mental problems. 

While the findings, published in the International Journal of Epidemiology, found an association between longer pregnancies and ADHD and other mental problems in children, it did not prove causality.

Additional research is needed to determine the reason behind the link, and to determine whether the link between post-term birth to emotional and behavioral problems continues past three years of age, lead researcher Hanan El Marroun said in a journal news release.

Researchers offer some explanations to the findings like post-term babies are often born larger, which has been associated with a greater risk of perinatal problems, or that the babies who spend longer time in the womb are exposed to an “old” placenta which offers fewer nutrients and less oxygen that what is required by a full-term fetus, which can lead to abnormal development. Another possible explanation for the emotional and behavioral problems associated with late births could simply be caused by a more complicated post-term birth process which could increase the child’s susceptibility to emotional and behavioral problems later in life.

Neena Modi, a professor of neonatal medicine at Imperial College London and the Royal College of Pediatrics, told The Guardian that the latest findings were "perfectly plausible [because] we know that birth after your due date is associated with a whole range of problems. If your baby stays in the womb for too long they are more likely to be a stillbirth, or weigh too little or be more likely to have a neurological disorder, because the longer a baby stays in the womb the more likely the placenta is to stop functioning normally."

"This research is interesting and important, and reinforces that the current guidance is well-based. It should, though, perhaps lead to a greater awareness among women about their risk of prolonged pregnancy. If they are offered to be induced, they should take it. If that's what their obstetrician is saying, that's good advice," Modi added.