Carriers of X-linked juvenile retinoschisis (RS) were previously suggested to give birth to an excess of boys. We determined the carrier status for the 214G > A mutation of the RS1 gene in 202 females belonging to a large RS founder pedigree. The secondary sex ratio (SSR) in the offspring of 149 carriers was 129.8 (z = 2.25), which differed significantly from that of the Finnish population (SSR 106) but not from that of 53 non-carrier females belonging to the same pedigree (SSR 116.7; z = 0.51). Since possible causes for the skewed SSR include factors affecting fertilisation, implantation and embryonic death, we searched for expression of RS1 in various placental and uterine cells and found that, in addition to the retina, RS1 is expressed in the uterus. We hypothesize that the RS1 protein has a role in implantation or embryonic survival.