A form of IVF has been linked to an increased risk of low intelligence in children.
The direct injection method, known as Intra-Cytoplasmic Sperm Injection (Icsi), was also linked to a severe type of autism but only in multiple births.
Scientists in the Swedish study, which is published in the Journal of the American Medical Association, analysed data on more than 2.5 million births.
They pointed out that the risk in real terms of an IVF baby being affected by intellectual impairment is tiny.
Compared to "natural" conception, IVF overall had no effect on autism rates and was linked to a very small 18% increased risk of low IQ, which appeared to be linked to multiple births.
But the researchers identified a 51% increased risk of intellectual impairment - defined as an IQ under 70 - in children conceived by IVF treatments in which sperm cells are injected directly into eggs.
It affected 92 per 100,000 (0.092%) children in the IVF group as opposed to a standard risk of 62 per 100,000 children (0.062%).
Icsi IVF was initially developed to assist infertile men but it now accounts for half of IVF treatments in the UK including those resulting from female fertility problems.
Copyright Press Association 2013