A third of babies born between 29 and 33 weeks still need specialist care aged five, a French study suggests.
Premature babies can need extra healthcare through childhood
Pregnancies should last about 40 weeks, and it is known that very early birth can lead to physical problems or learning difficulties in childhood.
But the Lancet study, of 400 full-term babies and 1,800 born before 33 weeks, found this later stage of prematurity still meant significant needs.
UK charity Bliss said the study showed the need for good follow-on care.
In 2005, just over 11,500 babies were born at under 33 weeks' gestation in England and Wales - with most of those born between 29 and 33 weeks.
The number of premature babies surviving is increasing as healthcare improves.
But the higher survival rate is raising issues about an increase in the rate of adverse developmental outcomes.
Questions and costs
The researchers from the Inserm Research Unit on Perinatal Health and Women's Health, Villejuif, France, and Université Pierre et Marie-Curie-Paris compared 1,800 babies born before 33 weeks with 400 babies born at full-term.
The so-called Epipage study then looked at the children when they were five years old, checking their physical health and tests of their memory and understanding.
Disabilities were also classed as minor, moderate or severe.
Rates of disability were highest amongst those born before 28 weeks, with 49% - 195 babies - affected.
But the actual number of children with disabilities was higher in the group born between 29 and 33 weeks, at 441 - or 36%.
And a similar pattern was seen when researchers looked at use of specialist health service such as physiotherapy, psychology, occupational therapy or day centres for the most-severely disabled.
Such resources were used by 42% of children born at 24-28 weeks but 31% of those born 29-32 weeks - compared with only 16% of those born at 39-40 weeks.
Writing in The Lancet, the researchers say: "These results raise questions about health and provision of rehabilitation services, and the cost of these services to families and society.
"Further work is needed to identify the best and most effective early developmental interventions to improve the functional prognosis of motor disabilities.
"As they grow older, children with cognitive deficits will have difficulties at school and will need help or special education."
But they said more research was needed on what interventions could help children's learning and understanding as they develop.
Dr Mary Jane Platt, a public health specialist at the University of Liverpool, UK, said the level of disability was important, but added: "There is quite a significant group that have some sort of cognitive impairment that is not a disability.
"We need to know more about how these impairments affect these children and how to support them."
She added: "The study reminds us that children born before 33 weeks need care and support that lasts far beyond discharge from the neonatal care unit."
A spokeswoman for the premature baby charity Bliss, said it was encouraging that 61% of the children followed in this study were reported to have no disability at all.
She added: "In the cases where children did experience some kind of disability in relation to their prematurity, this emphasises the need for adequate follow-on care to support these children as they grow."