Babies born extremely prematurely (search) may have a better chance of survival today thanks to advances in modern medicine, but new research suggests that the vast majority will develop some form of mental or developmental disability by age 6.

In the largest study to date of babies born at 22 to 25 weeks of pregnancy and followed through early school age, researchers found 41 percent have severe or moderate mental impairment at 6 years of age, compared with only 2 percent of their peers who were born full term.

Even more disturbing, experts say, is the finding that only about 20 percent of the children born extremely prematurely had no evidence of mental or developmental disability by age 6.

Researchers say studies from the late 1970s showed that extremely preterm children did not seem to fare much worse than extremely low-birth-weight children at a similar age.

But this study showed that compared to a more relevant group -- their classmates who were born full term -- extremely preterm children were almost twice as likely to have mental disabilities such as problems with mental processing, knowledge of facts, language concepts, and school-related skills.

The findings appear in the Jan. 6 issue of The New EnglandJournal of Medicine.

Disability Prevalent Among Premature Babies

In the study, researchers followed 241 children who were born before the 26th week of pregnancy in the U.K. and Ireland in 1995 and had survived to age 6. They assessed their mental and developmental abilities and compared them to a group of 160 classmates who were delivered at full term.

The results showed the prevalence of severe, moderate, and mild disability was at 22 percent, 24 percent, and 34 percent, respectively. In addition, disabling cerebral palsy was found in 12 percent of the children born very prematurely.

None of the 160 comparison children had cerebral palsy and only two had a moderate disability.

Researchers found the proportion of very premature children who had a severe disability at age 6 was similar to that at 30 months of age. But the study showed that differences between the sexes became more pronounced at age 6, with boys having a higher risk of disability.

Although this study was conducted in the UK, experts in the U.S. say the findings are also applicable to this country.

"Prematurity is a common, serious problem in America, and unfortunately, the number of preterm births is rising each year," said Scott D. Berns, MD, vice president for chapter programs of the March of Dimes, in a news release. "Too many babies are born extremely premature in this country, and the result is that many of them die in the hospital or suffer lifelong consequences, including cerebral palsy (search), mental retardation (search), chronic lung disease (search), blindness, and hearing loss."

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Marlow, N. The New England Journal of Medicine, Jan. 6, 2005; vol 352: 9-19. News release, March of Dimes.