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For premature babies, faster weight gain in the first year of life translates to a higher IQ at school age, researchers found.

The gain in cognitive ability comes with a catch, however -- an increase in blood pressure and consequent cardiovascular risk, according to Mandy Belfort, MD, of Children's Hospital Boston, and colleagues.

But the cognitive benefit appears to outweigh the cardiovascular risk, they reported in the June issue of Pediatrics.

One implication, the researchers concluded, is that increased nutritional support for preemies after they leave the neonatal intensive care unit "might benefit long-term neurodevelopmental outcomes, with only a small effect on (blood pressure)-related health."

Belfort and colleagues noted that slow weight gain in preemies has been associated with impaired neurodevelopment, but the risks or benefits of rapid weight gain have not been widely known.

To help fill the gap, they looked at 911 participants in an eight-center longitudinal study of children born at no more than 37 weeks' gestation and with a birth weight of no more than 2,500 grams (5.5 lbs).

The infants were weighed at term and at four and 12 months' corrected ages. Their blood pressure was measured three times when they were 6.5 years old. And they took the Wechsler Intelligence Scale for Children at age 8.

Overall, the median birth weight was 1.87 kilograms, ranging from 0.54 to 2.50, and the median gestational age was 34 weeks, ranging from 25 to 37.

The average systolic blood pressure at age 6.5 was 104.2 millimeters of mercury, and the average Wechsler score at age 8 was 91. Systolic pressure was measured because it is more predictive of later outcomes than diastolic.

The children were below average for weight, length, and head circumference throughout their infancy, compared with children born at term, but had caught up by age 8, they said.

The researchers calculated z scores for the infants' weight, where a z score of zero is the median for the reference population. At 12 months, the median z score for the infants was minus 0.7.

After adjusting for the child's sex, age, and race and the mother's education, income, age, IQ, and smoking, the researchers found that for each z score additional weight gain from term to 12 months:

• Systolic blood pressure was higher by 0.7 millimeters of mercury.
• The total score on the Wechsler scale was 1.9 points higher.

The researchers said that the IQ gain is not clinically important for individuals, but "shifting the IQ curve of a population upward by a few points can have an important impact."

The study's strengths include its large size and the fact that it included all premature infants, not just those born extremely early. The researchers were able to control for a range of possible confounders and could assess short periods of weight gain in infancy.

On the other hand, they cautioned, neonatal intensive care unit practice has changed since the 1980s, when the infants were born. The mothers were also from the lower end of the socioeconomic spectrum, which may limit how widely the findings apply.

Plus, the study may underestimate the cardiovascular risk, because the researchers did not have measurements of insulin resistance or other risk factors than blood pressure.