Step 1: Understanding Section 13 of the Transfer of Property Act.
Section 13 allows transfer of property for the benefit of an unborn person, provided the whole of the remaining interest is transferred.
Step 2: Nature of interest on birth.
When the unborn person is born alive, the interest created in his favour takes effect immediately, even if the enjoyment of the property is postponed.
Step 3: Distinction between vested and contingent interest.
A vested interest is not dependent on a condition precedent; only enjoyment may be deferred. In contrast, a contingent interest depends on fulfillment of a condition.
Step 4: Conclusion.
Since ‘Y’ acquires a present right on birth though enjoyment is postponed, the interest is vested interest.