The Restatement (Second) of Property - Donative Transfers § 1.4 illus. 5 says, in material part:

"5. O, owning Blackacre in fee simple absolute, makes an otherwise effective devise thereof “to A for life, then to A's first-born son for life, then to the issue then living of A's first-born son, such issue to take per stirpes.” A survives O, but has no son born at the time of O's death. The life interest in A's first-born son is subject to the unfulfilled condition precedent of his birth. His interest is vested as soon as he is born even though his interest might expire before he ever comes into possession of Blackacre, which would be the case if he should die before A."