Certain forms of IVF treatment are significantly associated with an increased risk of low intelligence in children, a major study has shown.
A link was also found with an especially severe type of autism, but only in the case of twins or triplets.
Scientists who analysed data on more than 2.5 million births stressed that the chances of an IVF baby being affected remained tiny in real terms.
They found a 51 per cent increased risk of intellectual impairment, marked by an IQ below 70, in children conceived by IVF treatments in which sperm cells are injected directly into eggs.
This amounted to a rise from 62 per 100,000 children (0.062 per cent) to 92 per 100,000 (0.092 per cent).
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.