For certain single-gene disorders, DNA probes allow individuals to make more informed decisions about family size and the outcomes of pregnancies. They may also have important psychological effects. We present a method of assessing their impact on the outcomes of pregnancy. We conclude that the traditional approach, which focuses solely on the potential of prenatal diagnostic services to reduce affected births, is limited. It neglects the potential of such services to promote an increase in unaffected births by reducing the number of unnecessary terminations.