Why does the risk of an autosomal recessive disorder stay the same in each pregnancy?

The risk of an autosomal recessive disorder remains constant for each pregnancy because the inheritance of genes is an independent event at every conception.1

In cases where both parents are carriers of a recessive allele, each parent contributes one copy of the gene to the offspring. Because the genotype of the parents does not change, the probability of them passing on the mutant gene remains the same for every child they conceive, regardless of the health outcomes of their previous children. For two carrier parents, this results in a 25 percent statistical risk that any future child will inherit two mutant alleles and be affected by the disorder.231