Iowa Code § 450.96
Estates in expectancy which are contingent or defeasible and in which proceedings for the determination of the tax have not been taken or where the taxation has been held in abeyance, shall be valued at their full, undiminished value when the persons entitled to the estates come into the beneficial enjoyment or possession of the estates, without diminution for or on account of any valuation previously made. When an estate, devise, or legacy can be divested by the act or omission of the legatee or devisee, it shall be taxed as if there were no possibility of the divesting. When a devise, bequest, or transfer is one in part contingent, and in part vested so that the beneficiary will come into possession and enjoyment of a portion of the inheritance on or before the happening of the event upon which the possible defeating contingency is based, a tax shall be imposed and collected upon the bequest or transfer as upon a vested interest, at the highest rate possible under this chapter if no contingency existed; provided that if the contingency reduces the value of the estate or interest taxed, and the amount of tax paid is in excess of the tax for which the bequest or transfer is liable upon the removal of the contingency, the excess shall be refunded as provided in sections 450.94 and 450.95 in other cases.
Iowa Code § 450.96
83 Acts, ch 177, §35, 38