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% 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%) to 92 per 100,000 (0.092%).
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.
Of the total number of children born between 1982 and 2007, almost 31,000 were conceived using IVF. They were followed up for an average of 10 years until 2009.
Of the 16,000 children diagnosed with intellectual impairment, 180 were IVF babies. Seven thousand children developed severe autism, of which 103 were born after IVF.
Compared with “natural” conception, IVF overall had no effect on autism rates and led to a very small 18% increased risk of low IQ which appeared to be linked to multiple births.
The significant findings only emerged when researchers compared six different types of IVF involving the standard “mixing-in-a-dish” method of fertilising eggs or Icsi.
Icsi used with fresh or frozen embryos produced 51% more intellectually impaired children than standard IVF.
Study leader Dr Avi Reichenberg, from the Institute of Psychiatry at King’s College London, said: “Our study shows that treatments developed to manage male infertility are associated with an increased risk for developmental disorders in offspring.
“The exact mechanism is unclear, but there are a number of risk factors, from selection of IVF procedures, to multiple embryos, and to pre-term birth. Whilst intellectual disability or autism remain a rare outcome for IVF, being aware of the increased risk associated with specific types of IVF means offspring at risk can be identified and potentially monitored for developmental disorders, ensuring they receive early detection and appropriate support and care.”
The researchers insisted the research should not hinder childless couples seeking IVF treatment.
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