Babies born just a few weeks early face higher odds of developmental delays and behavior problems that show up in kindergarten, a study of nearly 160,000 children found.
The research adds to emerging evidence about potential threats facing otherwise healthy "late preterm" infants, who until recently were not even called premature.
It also bolsters arguments against scheduling cesarean section births more than a few days early just for the convenience of doctors and mothers.
Babies in the study born at 34 to 36 weeks were 36 percent more likely to have developmental delays including learning difficulties in kindergarten than those born during the 37th to 41st week of pregnancy, which is the range for a full-term pregnancy.
Slightly more than 4 percent of late preemies had developmental delays, versus nearly 3 percent of the other infants. That would mean about 14,000 of about 360,000 late preterm children born each year are affected.
While that excess risk is notable, it should not raise undue alarm because most late preemies had no developmental problems, said lead author Dr. Steven Morse of the University of Florida.
Still, Morse said obstetricians, pediatricians and parents should be aware that these babies might face delays because "the sooner we can recognize it, the more likely we can do something about it."
Reasons for the results are uncertain, although brain immaturity is one theory.
Late preemies were slightly more likely to have low-income, less educated mothers, but Morse said factoring in those differences didn't change the results.
Brain may still be maturing
Other studies have suggested that the brain continues to mature in the final month of pregnancy, and that babies born a month early have smaller brains than those born on time.
The study was released Monday and appears in April's Pediatrics journal.
Until recently, babies born up to a month early were referred to as "near-term" infants, but recent studies have shown that they may develop problems shortly after birth that are similar to those affecting very premature babies, though not nearly as severe. These include breathing difficulties, problems regulating body temperature and jaundice.
Those problems usually require newborns to remain hospitalized for several days. The new study involved only late preemies sent home within three days of birth, who were presumed to be otherwise healthy.
The researchers compared Florida public school records for 7,152 children born healthy but slightly preterm with those of 152,661 youngsters born full-term.
Besides more developmental delays, the preterm children also were more likely to be suspended from school and to be held back from first grade.
The study didn't examine why babies in the study were born early; the most common reasons for infants generally include doctors inducing labor because of bleeding or blood pressure problems in the mother, and premature labor of unknown causes.
Sometimes doctors can use medicine to stop premature labor if the health of the mother and baby aren't at risk.
Dr. Helen Kay, obstetrics chief at the University of Chicago Medical Center, said the study results may make doctors more cautious about inducing labor.
"Maybe we aren't doing them a favor" by delivering babies a few weeks early, she said. "Let's pause, think" and perhaps wait, she said.