Even for infants born full-term, a little more time in the womb may matter.
The extra time results in more brain development, and a study suggests perhaps better scores on academic tests, too.
Full-term is generally between 37 weeks and 41 weeks; newborns born before 37 weeks are called premature and are known to face increased chances for health and developmental problems.
The children in the study were all full-term, and the vast majority did fine on third-grade maths and reading tests.
The differences were small, but the study found that more kids born at 37 or 38 weeks did poorly than kids born even a week or two later.
The researchers and other experts said the results suggest that the definition of prematurity should be reconsidered.
The findings also raise questions about hastening childbirth by scheduling caesarean deliveries for convenience - because women are tired of being pregnant or doctors are busy - rather than for medical reasons, the researchers said.
"[Women should] at least proceed with caution before electing to have an earlier term birth," said lead author Dr Kimberly Noble, an assistant paediatrics professor at Columbia University Medical Centre.
The study involved 128,000 New York City public school children and included a sizable number of kids from disadvantaged families. But the authors said similar results likely would be found in other children, too.
Of the children born at 37 weeks, 2.3 per cent had severely poor reading skills and 1.1 per cent had at least moderate problems in maths. That compares to 1.8 per cent and 0.9 per cent for the children born at 41 weeks.
Children born at 38 weeks faced only slightly lower risks than those born at 37 weeks.
Compared with 41-weekers, children born at 37 weeks faced a 33 per cent increased chance of having severe reading difficulty in third grade, and a 19 per cent greater chance of having moderate problems in maths.
"These outcomes are critical and predict future academic achievement," said Naomi Breslau, a Michigan State University professor and sociologist.
Her own research has linked lower IQs in six-year-olds born weighing the same as the average birth weights at 37 and 38 weeks' gestation, compared with those born heavier.
The study was published today in Pediatrics.
The research would cause quite a stir, said Dr Judy Aschner, a paediatrics professor and neonatology director at Vanderbilt University Medical Centre."There are still a lot of babies who are being delivered more or less electively at 37 and 38 weeks, with people thinking, 'This is no big deal - these babies are full-term.' I think this is a big deal," Dr Aschner said.
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