TUESDAY, July 2 (HealthDay News) -- Despite some concerns, children born by in vitro fertilization do not seem to have an increased risk of autism, a large new study finds. They may, however, have a slightly higher-than-normal chance of being intellectually impaired.
The study, reported in the July 3 issue of the Journal of the American Medical Association, looked at more than 2.5 million infants born in Sweden between 1982 and 2007. It found that the nearly 31,000 children conceived via in vitro fertilization (IVF) did not have an increased risk of the developmental disorder autism.
They were, however, 18 percent more likely to have an intellectual disability (which used to be called mental retardation), defined as an IQ lower than 70 and limited abilities in schoolwork .
Experts stressed that the risk is quite low: The rate of intellectual disability among IVF kids was about 46 per 100,000 each year, versus about 40 per 100,000 among kids conceived naturally.
"The vast majority of children born after the different types of IVF treatment will be perfectly healthy," said lead researcher Sven Sandin, of the Karolinska Institute, in Stockholm, and King's College London, in England.
What's more, the risk seemed largely related to the fact that babies born via IVF are often multiples (such as twins or triplets), and, therefore, frequently born preterm or at a low weight. That in itself carries a higher-than-normal risk of intellectual disability.
Experts said the findings suggest that whenever possible, IVF should involve implanting only one embryo in the woman's uterus, rather than the traditional route of implanting at least two.
"From the results of this study, we think that the use of single-embryo transfer should be extended," Sandin said.
An infertility specialist not involved in the study agreed. "I think we should be encouraging more single-embryo transfers," said Dr. Marcelle Cedars, director of reproductive endocrinology at the University of California, San Francisco.
That is happening more often these days, said Cedars, who wrote an editorial published with the study. At her center, she said, more than half of IVF patients have a single embryo implanted -- although that is higher than the national norm.
There is still a chance, however, that certain IVF procedures carry a risk.
Sandin's team found that a specific IVF technique used for male infertility -- called intracytoplasmic sperm injection, or ICSI -- was related to an increased risk of intellectual disability, even among single babies.
Again, the actual rates of intellectual impairment were quite low, Sandin said. But the results suggest there could be something about the intracytoplasmic sperm injection, or fathers' infertility, that contributes to the risk.
"People have for some time been concerned about the ICSI procedure," Cedars said.
That is partly because intracytoplasmic sperm injection is invasive: A single sperm is injected directly into an egg, whereas in standard IVF, sperm fertilize the eggs in a lab dish. "[ICSI] also bypasses the natural selection of sperm" that happens during fertilization, Cedars said, in which sperm compete to penetrate the egg and the fittest one wins.
But an alternative explanation is that male infertility -- which may involve genetically abnormal sperm -- is raising the risk of intellectual disability, rather than some effect of intracytoplasmic sperm injection, Cedars said.
Sandin agreed, but said it's impossible to say for sure based on these findings.
The study corroborates past research that has found no excess risk of autism after IVF. But this is by far the largest and best designed study to look at the question, Cedars said.
"This gives us powerful evidence that there is no association with autism," she said. She added that expectant parents can also be reassured that the risk of intellectual disability linked to intracytoplasmic sperm injection is still very low, even if it's higher than the norm.
Although the study found an association between certain IVF procedures and a higher risk of intellectual disabilities in children, it did not establish a cause-and-effect relationship.
Cedars and Sandin both said more studies are needed to see how children fare after IVF, and to dig for reasons for the current findings on ICSI.
Between 1978 and 2012, about 5 million children worldwide were born via IVF, and the numbers will only increase, Cedars said. And in some countries -- including the United States -- intracytoplasmic sperm injection is being increasingly used in cases in which there is no clear problem with the man's fertility, because there is a perception that it's more efficient.
But that belief, Sandin's team said, is unproven.
The American Society for Reproductive Medicine has more on in vitro fertilization and intracytoplasmic sperm injection.