What causes autism? Truth is, even after years of research, health experts still are unclear about exactly what causes autism and why some children are more susceptible to the disease than others. But two new studies, presented Wednesday at the International Meeting for Autism Research in Philadelphia, provide some of the strongest evidence to date that the condition may be linked to whether or not a woman undergoes fertility treatment to conceive.

The first study was conducted by a team at the Harvard School of Public Health. The study found that autism was nearly twice as common among the children of women who were treated with Clomid and other similar drugs that help to induce ovulation than women who did not suffer from infertility. The link persisted even after researchers accounted for the woman's age. The link only got stronger with exposure — the longer a woman was treated for infertility, the higher the chances her child would have autism. The study involved data from 3,985 women, of which 111 reported having a child with autism.

The second study, presented by an Israeli team, found an association between autism risk and in vitro fertilization (IVF), which also involves the use of drugs that stimulate ovulation. This study was smaller than the Harvard study, analyzing data from 564 children with autism, who had come to an autism center for in-depth evaluation. Researcher found that 10.2 percent of the children were the product of IVF, much higher than the rate in the general population of Israel, which is 3.5 percent.

There are still plenty of questions surrounding both studies, such as how many of the children were born prematurely or were twins or triplets, or had a low birth weight. These factors are all associated with infertility treatment and they are all also associated with the risk of autism.

But together, these studies add to a growing body of evidence that infertility treatment methods could play a role in causing autism.