The prenatal environment - new FAS study

Fetal Alcohol Syndrome has been identified as being associated with ASD risk:

http://www.ncbi.nlm.nih.gov/pubmed/9344050?

Classical twin study design assumes an equal prenatal envornment and twin studies in ASD have reported a 60% concordance rate in identical twins and 0% concordance rate in fraternal twins.

http://www.ncbi.nlm.nih.gov/pubmed/7792363?

The prenatal environment in twin pregnancies differs between identical and fraternal twins. 2/3's of identical twins share the same prenatal environment (same placenta) while 1/3 of identical twins and all fraternal twins develop in seperate placentas.

Could concordance rates in ASD classical twin studies be better explained by differences in the prenatal environment?

A new study suggests that the answer is yes. Gerari et al examined the placenta of a pair of fraternal twins who had been heavily exposed prenatally to alcohol. A series of guinea pig littermates who were prenatally exposed to alcohol was included in the study as a control.

The authors found that one fraternal twin had high amounts of alcohol in the placental tissue while the co-twin had no level of alcohol in the placental tissue.

http://www.ncbi.nlm.nih.gov/pubmed/19127948?

Classical twin studies which assume an equal prenatal environment may have overstated the magnitude of the genetic component in ASD.