Because the concordance rate between identical twins is only 88%, an environmental factor must cause autism spectrum disorder (ASD). Furthermore, when identical twins share ASD, it is to varying degrees suggesting different prenatal environments exist, which occurs when identical twins have separate placentas (~30% of the time). Placental inclusions are predictive of ASD along with excessive increases in extra-axial cerebral spinal fluid (CSF) detected by MRI in the brains of 6- and 12-month-old infants later diagnosed at 2 years with ASD. The human papillomavirus (HPV) can infect the trophoblast cells of placentas and transmit to the fetus where it infects the epithelial cells of the choroid plexus, a centrally located lining inside the brain responsible for producing CSF via the SLC4A10 gene product. HPV causes epigenetic changes, deletions, and duplications of genes, and besides its characteristic methylation patterns, the SLC4A10 gene was found to be increased in children with ASD. Moreover, male placentas implant close to the cervix (low-lying) three times more often than female placentas paralleling the ASD ratio of ~3:1 (boys to girls). Finally, the Australian HPV vaccination programme that began in 2007 might explain why the 0-4 yr. ASD incidence did not increase from 2010 to 2015.