It is my understanding that such marriages are fraught with risks for the children born of such unions. However, I am not a medical expert; you should, therefore, refer to the experts.
As Muslims, we are encouraged not to expose ourselves to unnecessary risks. If therefore the person you are marring may pose such risks, it is best to avoid such a marriage in the first place rather than worrying about complications in pregnancy, birth deformities, abortion, etc.
Abortion is not allowed in Islam; it is akin to murder after the pregnancy has advanced 120 days; before that it is less sinful and it should only be resorted to in extreme circumstances; for details, you may search answers posted on this site earlier: here is the link to one of them:
Abortion for Fear of Negligence: Permissible?
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