TUESDAY, April 19 (HealthDay News) -- The risk that children will develop attention-deficit/hyperactivity disorder (ADHD) rises with every week they are born short of full term, a new study suggests.

Earlier studies have shown an association between a too-early birth and the increased risk for ADHD. This study adds to that data by looking at the risk based on how preterm the delivery is, the Swedish researchers say.

"Our study is the first to report that the risk for ADHD is 40 to 60 percent higher in babies born moderately preterm," said lead researcher Dr. Anders Hjern, an adjunct professor of pediatric epidemiology at the Center for Health Equity Studies at the Karolinska Institutet in Stockholm.

"Even in babies born in the early term period -- at 37-38 weeks -- the risk is 20 percent higher," he added.

This underlines the fact that preterm birth carries significant risks and needs to be given more attention in neonatal care, and in follow-up within health care systems, Hjern said.

"The finding that early term birth carries an increased risk for ADHD has [especially] important implications for planned caesarian births, which are often performed during these very weeks," he added. "To minimize the risk for ADHD, these births should be planned as close to the full term date -- that is, week 40 -- as possible."

The report is published in the April 18 online edition of Pediatrics.

For the study, Hjern's team collected data on almost 1.2 million Swedish children born between 1987 and 2000. These children were followed to see if they took medication for ADHD when they were between 6 to 19 years old.

The researchers found that the earlier the birth, the greater the odds the child would develop ADHD. The uptick in risk ranged from between 10 to 20 percent for children born at 37 to 38 weeks of gestation, to 40 percent for those born at 33 and 34 weeks, and 60 percent for those born at 29 to 32 weeks of gestation.

Children born very preterm -- 23 to 28 weeks -- had double the odds of a full-term baby of developing ADHD.

Commenting on the study, Dr. Charles R. Bauer, professor of pediatrics at the University of Miami Miller School of Medicine, said that, "the lesson to be learned here is that late preterm babies are at risk."

These findings "argue very strongly against elective caesarian surgery, which is going on like crazy in this country," he said. "There are neurological issues, there are developmental issues, there are cognitive issues and now there are behavioral issues."

Bauer thinks women should not have caesarian delivery unless it is medically necessary. "A full-term baby was meant to be full term, and they shouldn't interfere with that unless it's necessary," he said.

More information

For more information on ADHD, visit the U.S. National Institute of Mental Health.