A term of 37 to 41 weeks is considered ''normal,'' but the new research finds birth at 39 to 41 weeks provides more developmental advantages compared to birth at 37 to 38 weeks.
"If the pregnancy is going well, it would be better to avoid doing elective C-sections early in the full-term window," said study author Dr. Betsy Lozoff, a professor of pediatrics and communicable diseases at the University of Michigan.
She and her colleagues tested 1,562 infants when they were 1 year old, then looked back to see at what week of term they were delivered. The babies were all born in Chile and all were delivered within the full-term window. Their average birth weight was 6.6 pounds.
For every additional week in the womb, however, the mental developmental test scored increased very slightly, by 0.8. The psychomotor scores -- which relate to body movement and coordination -- increased by 1.4 points for every additional week. This held after the researchers accounted for birth weight, gender, socioeconomic status and home environment.
The study is published online April 15 and in the May print issue of the journal Pediatrics.
The trends of early induction and early C-sections have increased to 40 percent of all births, according to the researchers. Because of how common they are, the study authors wanted to focus on the effects of brain development with early deliveries. According to the U.S. Centers for Disease Control and Prevention, the C-section rate reached 32 percent in 2007 in the United States.
The differences found in testing, on an individual level, were small, Lozoff said. The study reported average differences, so not all children born early were affected.
However, on a societal level, it could be very important, she said. "To give some reference point, the differences observed in this study are as large as those observed with low-level lead exposure." Exposure to lead has long been linked with developmental lags in children.
Although the study found an association between later delivery and a development advantage for infants, it didn't establish a cause-and-effect relationship.
The findings make sense, said Dr. Magaly Diaz-Barbosa, medical director of neonatology for Miami Children's Hospital. She hopes the new research will persuade doctors and women that those last few weeks in the womb are important.
"Even though the definition of a full-term gestation is between 37 and 41 weeks, what I think this study shows is, that each week in the pregnancy is crucial," she said.
"Some obstetricians might consider it OK to schedule a routine elective C-section after 37 weeks, but there is still important development [occurring]," she said.
Unless there is a medical indication or a risk, she said, it's better to wait until at least 39 weeks. The researchers suggested 40 or 41 weeks.
"There are still developmental changes, especially in the brain," going on late in pregnancy, Diaz-Barbosa said. "The healthiest place for the developing baby is in the womb."
For those parents whose baby was delivered early, study author Lozoff said they should be aware that their baby may lag slightly behind. "They shouldn't be surprised if the baby is a bit less mature in eating, sleeping and overall behavior," she said. "This will generally improve."
To learn more about developmental milestones for babies, visit the March of Dimes.