I think you're misunderstanding how the sentence I quoted relates to the study. The study I linked to actually only studied women who had abortions unrelated to fetal anomalies or life endangerment. So none of the 56% who declined to participate in the study nor any of the 44% who accepted to participate in it had abortions related to fetal anomalies or life endangerment.
The quote I quoted was basically an offhand remark that the paper made to justify its relevance, not backed up by evidence in the paper, but instead backed up by a citation to a different paper that I found hard to understand in a quick skim.
The general way I interpret the meaning of parentheses is that it's a part of the sentence that provides extra information, but the sentence would still be correct if it was removed.
Of course that doesn't invalidate the findings; but it is a a fairly small number of events they're talking about here, that's all.