In the largest study of its kind, researchers have confirmed that autism is twice as common among children born through assisted reproductive compared to natural conception. But they traced most of this increased risk to pregnancy and birth complications – especially those associated with multiple births (twins, triplets, etc.).
In fact, they found no increased rate of autism among children from assisted reproduction technology (ART) that resulted in a single birth.
"These results indicate that the higher autism risk may be due mainly to the large numbers of multiple births and complications of pregnancy and delivery among children conceived with ART," says lead researcher Christine Fountain, of Fordham University.
Addressing long-standing concerns
Assisted reproduction generally involves in vitro fertilization of one or more eggs that are then implanted in the mother’s womb. Assisted reproduction is associated with a number of factors known to independently increase a child’s risk of autism. These include having older parents; being a twin, triplet or other multiple; and birth complications including preterm delivery and low-birth weight.
A number of smaller studies had looked for associations between assisted reproduction and autism – both of which have increased dramatically in recent decades. However, their results have been mixed. Some showed an increased risk. Others did not.
To get a clearer picture, the new study looked at medical records related to nearly 6 million births in California between 1997 and 2007.
Overall the researchers found autism to be twice as common among children born through assisted reproduction than those who were not. However, this increased risk dropped considerably when the researchers accounted for advanced maternal age (over age 34) and medical complications during pregnancy or birth.
Even then, a small increased risk persisted for mothers 20 to 34 years of age. But the increased risk disappeared altogether when the researchers looked only at single births.
"The results indicate that reducing multiple births during ART may be beneficial for decreasing the risk of autism,” says co-author Dmitry Kissin, a reproductive-health epidemiologist with the Centers of Disease Control and Prevention.
In the past, it was standard procedure to transfer multiple embryos to the womb after in vitro fertilization – to increase the chances of successful pregnancy. However, the resulting multiple births increased the likelihood of a broad range of pregnancy and birth complications. In recent years, reproductive medicine societies have recommended a general reduction in the number of embryo transfers, and this has reduced rates of multiple births.