It is difficult to predict long term prognosis and life expectancy for women with Rett syndrome because it is such a rare disease, says Rettsyndrome.org. However, as of 2015, studies suggest that the life expectancy of most girls with Rett Syndrome is at least 47 years .
Girls with Rett Syndrome have a 95 percent chance of living to 20 to 25, compared to a 98 percent probability for the general U.S. female population, Rettsyndrome.org reports. This decreases to a 69 percent chance of reaching age 25 to 40, compared to a 97 percent chance for the general U.S. female population. Scientists are unable to make reliable estimates about life expectancy beyond age 40 due to the rarity of the condition and the relatively young age of most girls who have been diagnosed with the disease.