Children born after any in-vitro fertilization (IVF) procedure have a small increased risk of mental disability compared to spontaneous conception, according to a study published Wednesday in the Journal of the American Medical Association.

While mental disability remains a rare outcome of IVF, the findings provide "important evidence for parents and clinicians on the relative risks of modern IVF treatments for male infertility," said the study.

By using anonymous data from the Swedish national registers, researchers from Britain, Sweden and the United States analyzed more than 2.5 million children born between 1982 and 2007 to examine the association between use of any IVF and different IVF procedures and the risk of autism and mental disability.

The study found 30,959 of the children, or 1.2 percent, were born following IVF. Of the 6,959 diagnosed with autism, 103 were born after IVF; of the 15,830 with mental disability, 180 were born after IVF.

Overall, when compared to spontaneous conception, children born from any IVF treatment were not at an increased risk of autism, but were at an 18 percent increased risk of mental disability, the study said. However, the risk increase disappeared when multiple births, which were common in IVF, were taken into account.

The researchers also compared all six different types of IVF procedures available in Sweden and whether intracytoplasmic sperm injection (ICSI) was used. Developed in 1992, ICSI is recommended for male infertility and is now used in about half of all IVF treatments. The procedure involves injecting a single sperm directly into an egg, rather than fertilization happening in a dish, as in standard IVF.

Children born after IVF treatments with ICSI were at a 51 percent increased risk of mental disability, a rise from 62 cases to 93 cases per 100,000, it said.

Children born after IVF with ICSI were also at an increased risk of autism but the association disappeared when multiple births were taken into account, said the study.

"When we separated the different IVF treatments, we found that 'traditional' IVF is safe, but that IVF involving ICSI, which is specifically recommended for paternal infertility, is associated with an increased risk of both intellectual disability and autism in children," Sven Sandin, co-author of the study from King's College London's Institute of Psychiatry, said in a statement.

Karl-Gosta Nygren, co-author from Karolinska Institutet, however, said the majority of children are born perfectly healthy following IVF and that more research is needed to elucidate the reasons behind their findings.

It is estimated that approximately 5 million infants worldwide were born from IVF between 1978 and 2012.