Why does the chance of an autosomal recessive disorder remain the same in each pregnancy?

In the context of genetics, the probability of an autosomal recessive disorder remains constant across each pregnancy because the inheritance of genes is an independent event for every child.12

When both parents are carriers (heterozygotes, Aa), each parent has a 50% chance of passing either their normal allele or their mutant allele to their offspring. Because these alleles segregate independently during the formation of gametes, the outcome of one conception does not influence the genetic makeup of the next.12

For such a couple, the statistical probability for each pregnancy is:
* 25% (1 in 4): The child inherits the mutant gene from both parents and is affected (aa).12
* 50% (2 in 4): The child inherits one mutant gene and one normal gene, becoming an unaffected carrier (Aa).12
* 25% (1 in 4): The child inherits two normal genes (AA).12

Because these odds are recalculated at every fertilization, the risk does not "accumulate" or change based on the health of previous children.12