Rule against perpetuities
The unborn widow

The problem of the unborn widow is a frequently used illustration of the odd outcomes of the traditional rule against perpetuities. The unborn widow rule prohibits an unidentified widow from being treated as a validating life. If, for example, a grantor's will devised land "to my son, for life; then to his wife, for life; then to his children living at the time of her death", the children's contingent remainder (contingent on their status as "living" at the time of the widow's death) would be invalid, even if the grantor's son was an elderly and already-married man. Regardless of the age of the grantor's son, he could leave or lose by death his current wife and subsequently marry another woman who was not yet born at the time of conveyance; thus the widow that survived him would not be, with certainty, a life in being at the time of conveyance.
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