Researchers examined 41 studies conducted from 1990 to 2016. The studies included more than 2,700 extremely low birth weight babies (less than 1,000 grams, or just over 2 pounds) and more than 11,000 normal birth weight babies in the United States, Canada and 10 other countries. The extremely low birth weight babies had a higher risk for certain mental health problems. As children, they were more likely to have attention deficit hyperactivity disorder (ADHD), and as teens they had a greater risk of ADHD and social problems, the researchers found.