Premature babies given two drugs scored much better by age four on measures of intelligence, language and memory than preemies who didn't get them.

Two blood-building drugs injected soon after birth may give tiny preemies a lasting long-term edge, boosting brain development and IQ by age four, a first-of-its-kind study has found.

The study was small but the implications are big if larger, longer studies prove the drugs help even the playing field for these at-risk children, the researchers and other experts say.

Babies who got the medicine scored much better by age four on measures of intelligence, language and memory than preemies who didn't get it. The medicine group's scores on an important behaviour measure were just as good as a control group of four-year-olds born on time at a normal weight.

The results are "super exciting", said Dr Robin Ohls, the lead author and a paediatrics professor at the University of New Mexico. She said it's the first evidence of long-term benefits of the drugs when compared to no blood-boosting treatment.

Even though the treated youngsters didn't do as well as the normal-weight group on most measures, their scores were impressive and suggest greater brain development than the other preemies, Ohls said.

They scored about 12 points higher on average on IQ tests than the untreated kids but about 10 points lower than the normal-weight group. On tests measuring memory and impulsive behaviour, the treated kids fared as well as those born at normal weight.

The study involved 53 children, most white or Hispanic, born more than a month premature and weighing less than 1.7kg at hospitals in New Mexico, Utah and Colorado. Two dozen normal-weight children were also included.

Results were published on Monday in Pediatrics.

Shortly after birth the preemies were randomly assigned to receive injections of either erythropoeitin (EPO), three times weekly; darbepoetin once a week for several weeks; or no treatment. The drugs build red blood cells and are approved to treat anaemia caused by cancer treatment or resulting from other conditions.

Preemies lack the ability to make new red blood cells and often need frequent blood transfusions to replace blood taken for lab tests. The drugs are now sometimes used to try to reduce their need for transfusions, in doses similar to the ones studied.

The drugs can increase endurance by boosting oxygen levels in the blood, and have been implicated in some sports doping scandals.

Dr Sandra Juul, a University of Washington pediatrics professor, is leading a larger multi-centre study of both drugs in preemies and said it's too soon to recommend the medicines for treating developmental delays.

Still, since almost half of infants born extremely early have significant developmental problems, any treatment that could improve their lives "is incredibly important", Dr Juul said.