Following a review of 12 studies involving more than 1,700 participants, researchers have concluded that premature babies and those that born at low birthweight are more likely to develop ADHD than full-term, healthy-sized newborns.

Why this is the case is still not known, though researchers postulate that factors contributing to premature or underweight births might also contribute to ADHD. These include the mother’s medical history, as well as lifestyle factors during pregnancy such as smoking, eating, and drinking.

According to a report in Reuters, the study, published in the journal Pediatrics, compared full-term healthy babies to premature and underweight infants:

Researchers looked at data on healthy babies who were born weighing at least 2,500 grams (5.5 pounds) or arrived after 37 weeks’ gestation and they also considered smaller, earlier arrivals.Compared with these healthy babies, infants born at less than 32 weeks’ gestation or weighing less than 1,500 grams (3.3 pounds) were more than twice as likely to develop ADHD, researchers report in Pediatrics.When babies were born at less than 28 weeks’ gestation or weighing less than 1,000 grams (2.2 pounds) their odds of ADHD were more than four times higher than healthy infants, the study also found.

Thanks to the large number of participants, this robust study provides strong evidence of the “connection between an ADHD diagnosis and starting life too early or weighing too little,” said Joel Nigg, director of the ADHD and Attention Disorders Program at Oregon Health and Science University in Portland.