The heavier a baby is at birth, the bigger the brain power later in life. UK scientists say birth weight may have lasting effects on mental performance, influencing test scores and even educational achievements all the way to early adulthood.

In a study that followed 3,900 men and women born in 1946, investigators found that even among people born within normal weight range, bigger was better in terms of mental test scores during childhood, adolescence and young adulthood. Low birth weight -- less than about 5.5 pounds -- has been linked to delayed cognitive development, but little is known about the effect of birth weight in the normal range.

Investigators looked at the study participants' scores on tests of vocabulary, math and other skills at the ages of 8, 11, 15, 26 and 43. They also considered the participants' level of education by age 26. The researchers found that up until the age of 26, higher birth weight was related to better test scores. At age 43, birth weight was no longer important.

The link between birth weight and test performance held even when the researchers looked at factors such as parents' social class and education. In addition, Richards and colleagues found that individuals with higher birth weights also achieved higher educational or training levels.

These findings support earlier research suggesting that birth weight affects cognitive development, regardless of "social background," the authors write. The reasons for the connection are unclear, but the research team notes that birth weight is strongly related to head circumference at birth and, thus, brain size.

In addition, they write, birth weight is associated with certain growth factors that may influence the development of the central nervous system and cognitive ability.

13 British Medical Journal January 27, 2001;322:199-203