Is the failure to provide medical assistance to a deformed fetus that will inevitably die considered a sin requiring seeking forgiveness?

The reference for determining the cause of death is medical professionals. It was obligatory for you to adhere to the doctor's advice and accept medical assistance, as there was a possibility that the child could live, for some children born without a brain have lived for more than ten years.

Therefore, if the doctors provided the necessary assistance and the child died, then there is nothing upon you. However, if the child could have been saved but you refused assistance, leading to their death as attested by the doctors, then this is considered negligence and akin to unintentional killing.

In the case of unintentional killing, the blood money (diyya) is upon your 'aqilah (male relatives on the father's side), and you must offer expiation (kaffarah), which is the freeing of a slave. If you do not find one, then it is fasting for two consecutive months. You must also repent and seek forgiveness.