Is it permissible to abort a fetus before forty days have passed due to the husband's intense and unbearable sexual aversion?

It is not permissible to abort the fetus at any stage of its life, and your dislike for your husband is not a justification for that. You should entrust your matter to Allah, and supplicate Him to make your fetus a righteous child; because the supplication of parents is answered. And if a woman dislikes her husband, she has the right to seek khulʿ (divorce at the wife's instance).