Our primary hypothesis was that women who were randomized to informed free choice would undergo invasive prenatal diagnostic testing at a lower rate than women randomized to the usual care group, regardless of maternal age. Given that the restrictions on access to testing based upon maternal age had been eliminated, we hypothesized that women of all ages who were randomized to informed free choice would have lower invasive test use rates compared to women randomized to usual care. In our study of trends in the use of invasive diagnostic testing among women who delivered at an integrated health care system in California, we found that the number of amniocenteses and CVS procedures performed as a percentage of the total number of deliveries in 2006 was 12.1% (Norton ref). Assuming N=1200-381=819 participants for the new trial, 80% power, two-tailed alpha equal to 0.05, 90% retention at follow-up, and a 12.1% invasive test use rate within the usual care group, the design was capable of detecting a group difference corresponding to an invasive test use rate of ≤6.2% in the informed free choice group (OR=2.10, a 'small-medium' effect size). We also hypothesized that compared to usual care, women in informed free choice would have increased testing knowledge and risk comprehension as well as reduced decisional conflict, regret, and pregnancy worry; the design was capable of a minimum detectable a group difference equal to d=0.21 standard deviations, which is considered a 'small' effect size.