Birth order and parental age in microphthalmos and other ocular diseases.

We compared the distribution of birth order and maternal and paternal ages of blind school children throughout Japan with that of the total Japanese population of the corresponding age groups and with that of a subgroup of children with acquired blindness. The number of first-born children with microphthalmos was smaller, and the number of second-, third-, or fourth-born children was larger, as compared with the control groups. The differences were highly statistically significant by chi-square test. There was a less pronounced indication of birth order effect in amblyopia, congenital cataract, and optic nerve atrophy, which involved more first-borns than in the controls. The distribution of maternal age was also different from the control group in microphthalmos, congenital cataract, corneal opacity, and optic nerve atrophy. Less mothers in their 20s and more in their 30s produced children with these conditions. We believe this finding may be partly related to the rapid decline in infant mortality and in the incidence of congenital blindness in Japan.