First of all, the preamble creates no substantive, enforceable rights. Those are recognized only in the Bill of Rights and following Amendments.
Secondly, "procure blessings for our posterity" has in mind only the establishment of a civil order that will last for posterity. It is not intended to create any substantive rights.
The UNBORN are mentioned. And the preamble states the parameters of the constitution. Therefore, it cannot be interpreted outside those parameters.
The government cannot find in favor of the general demise of the population.
It cannot find in favor of the death of our posterity....our unborn.
Altogether simple, imho.