Rarity of Human Life

Recently I have been thinking about the odds of human life existing. I tried looking for several sources to try and find some sort of an answer, but time and time again, the answer I'm met with is it's too small to possibly calculate. I finally found a source that said a conservative estimate would be more than 1 in 10^2,000,000. I've been pro-choice basically all my life, but I was recently thinking about the fact that the percentage of fetuses with down syndrome that are aborted is 70%. If life is truly as rare as it seems, shouldn't everyone, despite having birth defects, have a right to experience it? A counterargument to that is that the babies would not know that they would exist, but couldn't you say the same thing with killing someone in a coma? By the way I don't have a firm opinion on any of this; this is just a thought experiment and I'm wondering what you guys think