Tiny “miracle babies” make for heartwarming stories, but the reality is that nearly half of all infants born extremely premature have significant learning and physical disabilities by the time they reach school age, the largest such study found.

Medical advances have allowed doctors to save earlier and smaller babies. While some developmental problems are known to be common among such children, the long-term consequences were not entirely clear.

“We needed to have some idea of really what this group was like when they grew up,” said one of the researchers, Dr. Neil Marlow, a neonatologist at the University of Nottingham in England.

Normal pregnancy is 37 to 42 weeks. Marlow and his colleagues looked at 241 children about 6 years old who had been born between 22 and 25 weeks. They found that 46 percent had severe or moderate disabilities such as cerebral palsy, vision or hearing loss and learning problems; 34 percent were mildly disabled; and 20 percent had no disabilities. Twelve percent had disabling cerebral palsy.