A more thorough explanation:

The presumption-of-fertility rule is a legal concept that assumes a woman can become pregnant as long as she is alive. This rule is used in the context of the rule against perpetuities, which limits the duration of certain types of property interests.

For example, suppose a testator leaves a bequest in trust to pay the income to A for her life, then to pay the income to the children of A for their lives, and upon the death of the survivor of such children, to pay the principal to the grandchildren of A. If A is 79 years old at the time of the testator's death and has two children, the remainder to the grandchildren may be invalid under the rule against perpetuities. This is because the law assumes that every person is capable of having children until the day they die, and therefore A could have an additional child who would not be a life in being at the testator's death, and that child could have a child who would be born beyond the permissible period.

The presumption-of-fertility rule is sometimes referred to as the "fertile octogenarian" rule, and it is based on the legal fiction established in the case of Jee v. Audley. This rule is used to determine whether a property interest violates the rule against perpetuities.

Overall, the presumption-of-fertility rule is a legal concept that assumes a woman can become pregnant as long as she is alive, and it is used to determine the validity of certain types of property interests under the rule against perpetuities.