Below, please find the answer by the head of the American Muslim Jurists Assembly, Sheikh Dr. Salah Al Sawy:
In the name of Allah, the Beneficent, the Merciful,
All praise is due to Allah and may peace and blessings be upon the Messenger of Allah, upon his household, his companions, and whoever follows him. To proceed:
It is not permissible to abort a malformed fetus if that is only discovered after four months have passed and it is known for certain that the soul has been blown into the baby. He has reached that stage, so he enjoys all the rights that all living beings enjoy. It is neither from Shari`ah nor reason to treat a weakling or a sick person by killing him. There are no exceptions to this rule, unless his presence poses a risk to the life of the mother, for we do not sacrifice the life of the origin [the mother] for the sake of the unborn child; the Fiqh Council in the Holy City of Mecca has issued a ruling to that effect. If, however, this deformation had been discovered before the passing of 120 days of gestation and it had been confirmed by the report of a medical commission of trustworthy, specialized physicians, based on professional examinations with technical equipment and lab work, that the fetus is severely and untreatable deformed, and that if it remains and is born on time, its life will be poor in quality and painful for both it and the family, then, in that case, it would be permissible to abort at the request of both parents.
And Allah Almighty is the Most High and He knows best.