But the researchers said the result could not be explained by factors such as premature and multiple births and needed further investigation.
The direct injection method, known as Intra-Cytoplasmic Sperm Injection (Icsi), was developed to help infertile men, but it now makes up half of IVF treatments in the UK including those resulting from female fertility problems.
In rare cases Icsi treatments are carried out using sperm that is surgically extracted. This procedure led to a more than four-fold increased chance of a child developing a severe and highly disabling form of autism.
The association vanished when multiple births were taken into account, leading scientists to suspect that some factor other than the Icsi procedure was responsible.
The Swedish study, published in the Journal of the American Medical Association, is the first of its kind to compare a wide range of IVF treatments.
Icsi used with fresh or frozen embryos produced 51 per cent more intellectually impaired children than standard IVF.
- PA