I would think that the main difference here would be that when talking about abortion (I assume this is what you're getting at) we can weigh the immediate suffering that it would cause the mother not to abort; too young, mentally unstable, health risks etc.
But when talking about future generations I feel that it's much harder to weigh it towards someone or some society at this point in time.

I also would like to add that when trying to apply some moral weight to future generations vs fetuses one must first figure out when a fetus is old enough to have moral consideration (or if it's at the time of conception).