Being born at below-normal weight is associated with a lower intelligence quotient (IQ) not only in childhood and young adulthood, but even at age 50, according to a Danish study.
Researchers found IQ differences between underweight and normal-weight babies remained stable into midlife, and even within the normal birth weight range, higher weights equated with slightly higher IQ throughout life.
"We found that the association between birth weight and intelligence is stable from young adulthood into midlife," said lead study author from the University of Copenhagen, Trine Flensborg-Madsen.
"There are long-term cognitive consequences of birth weight that do not diminish," Flensborg-Madsen said.
This association remained even after researchers adjusted for other factors such as household socioeconomic status and babies' gestational age.
Birth weights of less than 2.5 kilograms (5 pounds) have long been linked to a variety of health problems including the potential for a lower IQ in youth.
Compared to babies born at 2.5 kg or less, infants with a birth weight of 3.5 to 4 kg scored more than five points higher on IQ tests at age 28 and again at age 50, the study found.
For the study, researchers examined data on almost 4,700 babies born in Copenhagen from 1959 to 1961, including birth records and results from intelligence assessments done when participants were 19, 28 and 50 years old.
They sorted babies into five weight categories: underweight, meaning 2.5 kg or less; 2.5 kg to 3 kg; more than 3 kg up to 3.5 kg; more than 3.5 kg up to 4 kg; and overweight, or more than 4 kg.
Average-weight babies in the study fell in the middle category of more than 3 kg up to 3.5 kg.
Birth weight was significantly associated with intelligence at all three follow-up assessments, the researchers report in Pediatrics.
This association remained even after researchers adjusted for other factors that can influence intelligence such as household socioeconomic status and babies' gestational age. A low birth weight cannot explain all of this connection because the association also held up among babies born at a range of healthy weights, the authors also note.
The study was not a controlled experiment designed to prove that low birth weight directly causes lower IQ scores or that heavier babies are smarter.
Another limitation is the potential for many factors not explored in the study, such as home environment, maternal stress or parental intelligence, to influence babies' future IQ scores, the authors point out.
"The brain develops rapidly before birth, and therefore lower birth weight may reflect a poorer environment for brain growth," said researcher at the University of Edinburgh Dr Susan Shenkin, who was not involved in the study. "We still can't say whether low birth weight 'causes' lower cognitive ability test scores."