'A' transfers his property to 'B' for life and thereafter to his unborn son for life. In this case…

Section 13 of the Transfer of Property Act provides that an interest created for an unborn person "shall not take effect, unless it extends to the whole of the remaining interest of the transferor in the property". A mere life interest to the unborn son does not, so it is void; the illustration to section 13 gives this very result.

The life interest to B, a living person, is valid. The gift to the unborn son is not merely voidable, and "illegal" is not the term the Act uses.