Association Discovered in Identical Twins Only, Not in Fraternal Twins
A recent Swedish case-control study discovered that lower birth weight was significantly associated with neurodevelopmental conditions (NDCs), such as autism and intellectual disability (ID), among monozygotic twins. However, the same association was not found in dizygotic twins. Within the monozygotic pairs, the twin with a higher birth weight had fewer symptoms of autism and attention-deficit/hyperactivity disorder (ADHD), higher IQ ratings, and lower odds of being diagnosed with autism and ADHD. These findings suggest that as an indicator of fetal growth alterations, lower birth weight is a nonshared environment (NSE) factor for neurodivergent outcomes, including both symptoms or diagnoses of autism, ADHD, and ID. Since this link was observed only among the monozygotic twin pairs, a genetic confounding component of the association, besides the nonshared factor, was also apparent. The authors point out, given the link between low birth weight and future NDCs, it is important to acknowledge birth weight as influential when assessing identical twins’ neurodevelopment. Additionally, they suggest that it is essential to recognize early identification of factors with identical twins that contribute to fetal growth restriction, such as unequal placental sharing, unbalanced intertwin blood transfusion, or slow intertwin blood transfusion. If these conditions are discovered, fetoscopic laser treatments should be considered to minimize detrimental outcomes. The authors end their study by calling for more research to obtain greater knowledge and understanding of the associations between low birth weight and NDCs.
