All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Abortion is forbidden because it is a transgression upon a soul without any legal reason. Also abortion is a source of spreading corruption and destroying progeny and this is forbidden in the Sharee’ah except if there is a dire necessity for doing it such as fearing for the life of the mother if she keeps the fetus in her womb; only in such a case would abortion be permitted.
The case which the questioner mentioned is not a legal necessity or sound reason for abortion because Allah, who has created the fetus, is kinder to him than his own parents and all other people.
Allah's acts are the epitome of wisdom, hence He bestows health upon whomever He wishes of His slaves out of His kindness, and He tries whomever He wishes of His slaves with illness out of His justice. There is no resumption for His Judgment or resistance to His Destiny. Allah has the absolute power to heal the fetus because He is the One who descends the illness and He is the One who cures out His Grace.
Finally, the parents of the fetus should be pleased with Allah's Predestination and Decree and they should supplicate Him for good health for them and their fetus as Allah is the All-Hearer and He answers supplications.
Allah knows best.