Despite concerns that children born through Assisted Reproductive Technologies (ART) may develop differently from other kids, a U.K. study finds they have normal mental skills until at least age 11.

In fact, at ages 3 and 5 years, kids born as a result of these techniques had greater verbal cognitive ability than those born through natural conception, though this gap diminished with time. Researchers say that the older, better educated and more financially well-off parents of ART kids may play an important role in this difference at early ages.

“In the last decade, we’ve seen a huge increase in the use of ART and discussions about the outcomes and child development,” said lead author Anna Barbuscia, a sociology researcher at the University of Oxford.

The first “test tube baby” was born in the U.K. in 1978. And globally, about 5 million children have been conceived with ART since then, according to the U.S. National Institute of Child Health and Human Development.

“We’re finding that this way of birth is not linked to problems and, in fact, could show consistent better cognitive development in early childhood years,” Barbuscia told Reuters Health. “We were surprised about the results.”

She and her Oxford colleague Melinda Mills analyzed data on 15,218 children from the U.K. Millennium Cohort Study, which follows infants born in 2000 and 2001. All had been tested on vocabulary, verbal ability and reading at ages 3, 5, 7 and 11 years. Within this group, 186 children had been conceived through ART.

The research team found that children conceived through ART had higher verbal ability scores at ages 3 and 5 years, but the difference decreased over time and disappeared by age 11.

“We want to be very clear that our results don’t show that ART is better, but that it has no impact overall on cognitive abilities,” Barbuscia said. “We’re doing additional studies now to see what the comparison is after age 11.”

“It’s perhaps surprising that the differences do not persist, but as the authors note, other factors external to the home may have more influence as the children grow up,” said Claire Carson of the National Perinatal Epidemiology Unit at the University of Oxford.

Carson, who wasn’t involved in the current study, has conducted similar research on the same Millennium Cohort Study population and found no differences in cognitive abilities for 3- and 5-year-olds, as well as some evidence of mental health issues among ART kids at ages 7 and 8.

“Cognitive outcomes are only one area of research into child outcomes after fertility treatment that is being conducted globally,” she told Reuters Health by email. “There are still evidence gaps with regard to long-term health and development that need to be addressed.”