There is evidence for long-lasting negative effects of adverse birth characteristics such as low birth weight, short birth length, small head circumference, preterm birth, and reduced fetal growth on academic achievement and cognitive functioning in childhood and early adulthood, independent of familial factors and socioeconomic status.
In line with this, anthropometric measures (that are measures of the size and proportions of the human body, such as height and head circumference) in adulthood are positively associated with cognitive functioning throughout adulthood and negatively with age-related cognitive decline later in life. Finally, lower cognitive ability, lower education, and low occupational achievement throughout a person’s lifetime have been linked to an increased risk for age-related cognitive decline and dementia. This phenomenon — that individuals who are educated and have higher cognitive functioning are somewhat protected — is often referred to as cognitive reserve. Research suggests that cognitive reserve may be established quite early in life.
Together, these past findings made us wonder whether such risk/protective factors may at least partly be established even before birth, manifesting themselves in measures related to birth size, which would result in an association between birth characteristics and measures of age-related cognitive dysfunction spanning an entire lifetime. To our knowledge, no study to date has unequivocally established such a relationship.
What we have done
We conducted a registry-based cohort study using data from the Swedish Twin-, Birth-, Patient-, and Cause of Death Registers resulting in a sample of more than 35,000 individuals born between 1926 and 1960with information on birth characteristics, health status (dementia diagnosis), and vital status. A subsample of 4000 twins older than 64 years also participated in a cognitive screening. The main outcome measures were registry-based dementia diagnoses in the full sample and telephone-assessed cognitive impairment in the subsample.
Being born with low birth weight for gestational age as well as a small head for gestational age increased the risk for cognitive dysfunction late in life up to two-fold, even after controlling for familial factors, childhood socioeconomic status, and education in adulthood. In line with this, each additional gram birth weight and mm head circumference independent of gestational age was protective. Overall birth length seemed to be of less importance. By comparing identical twins who share all their genetic and familial environment (including prenatal factors) but were different in birth size, we could show that the observed association with cognitive dysfunction late in life is likely not entirely explained by familial confounding.
What this research shows
We showed that indeed smaller birth size (especially birth weight and head circumference adjusted and unadjusted for gestational age) poses a small but significant risk for late-life cognitive dysfunction. Further, it seemed that this effect may largely be independent of familial confounding as well as of measures of cognitive reserve in mid-life. The findings highlight the importance of preventing prenatal growth restriction and closely monitoring the cognitive development of infants of small birth size. Potential early life interventions targeted at increasing cognitive reserve need to be evaluated in future research.
These findings are described in the article entitled Associations between birth characteristics and age-related cognitive impairment and dementia: A registry-based cohort study, recently published in PLoS Medicine. This work was conducted by Miriam A. Mosing, Cecilia Lundholm, and Sven Cnattingius from the Karolinska Institutet, and Margaret Gatz and Nancy L. Pedersen from the Karolinska Institutet and University of Southern California Los Angeles.