Generally, but not necessarily.
As I implied before, a trust can be structured so that if one sibling predeceases another sibling while both have assets being held in trust, the trust assets will then be applied to the ebenfit of the surviving sibling and not the parent.
Aside from the angles that can be played using Trusts, what would be the case if it was a simple will... would the child, unborn at the time of the final division of property, be considered a beneficiary?