According to a research review, Babies who are born much too soon or who arrive weighing too little may not score as high on intelligence tests during childhood as full-term infants.

Researchers examined data from 71 previously published studies that included a total of 5,155 full-term babies and 7,752 of the most vulnerable preemies: those born at less than 32 weeks gestation or weighing less than 1,500 grams (3.3 pounds).

Overall, intelligence quotient (IQ) tests typically completed sometime from age 5 to age 20 showed that preemies typically lagged behind their full-term counterparts. On average, preterm youth underperformed on the IQ tests by the equivalent of about 13 points.

“The progress in neonatal health care that has been made since the 1990s considerably increased the survival of preterm infants,” said lead study author Sabrina Twilhaar of the Vrije Universiteit Amsterdam, Academic Medical Center.

“However, this improvement in survival was not accompanied by improvement of cognitive outcomes,” Twilhaar said by email.

Half of the preemies in the study were born before 28.5 weeks gestation and half of them arrived weighing less than about 1,000 grams (2.2 pounds)

Each additional week of pregnancy that babies missed with preterm deliveries were associated with a 1.26-point reduction in IQ scores, researchers reported online in JAMA Pediatrics.

Based on the lower scores for preemies in the study, researchers estimated that about 16% of the earliest arrivals would have a serious intellectual disability, compared with about 2.5% in the total population. Babies in the analysis were born between 1990 and 2008, and the results didn’t appear to differ based on year of birth.

Preemies who had bronchopulmonary dysplasia, who required long-term use of oxygen and mechanical ventilation, did appear to fare worse on IQ tests than preterm infants who didn’t receive these interventions.

This suggests that interventions to lower the risk of this lung disease could help improve cognitive outcomes for preemies, said Robert Joseph, a researcher at Boston University School of Medicine who was not involved in the study.

“The factors that contribute to the development of these illnesses are not completely understood, but probably include infections, oxidative stress, nutritional deficiencies arising from preterm birth, and therapies that are needed to assure the survival after very preterm birth, such as mechanical ventilation,” Robert said by email.

The study was not a controlled experiment designed to prove whether or how a premature delivery might directly cause intelligence deficits. Researchers also lacked data on certain demographic factors that might independently impact the odds of a low IQ.

Even so, parents of preemies should be aware of the potential for intellectual deficits because early interactions between parents and babies can help the brain develop cognitive skills, said Riikka Pyhala, a researcher at the University of Helsinki in Finland who wasn’t involved in the study.

Parents should also understand that IQ is an imperfect indicator of cognitive abilities or how well children might succeed in school, Pyhala said by email.

“Behind this general IQ score, every child has their individual strengths and weaknesses in more specific cognitive skills such as verbal or visual reasoning, working memory and attention,” Pyhala said. “For those children who have cognitive problems or difficulties in learning, detailed knowledge on these specific cognitive abilities gives us tools to support their learning in an optimal way.”