Answered by: Maulana Mohammed Dilwar Hussain
The patient had abnormal pain during pregnancy and continuously complained to the doctor, doctor thought that it was related to pregnancy so didn’t give it much thought.
Now they find out it’s stage 4 cancer.
Can they sue the doctor?
In the name of Allah, the Most Gracious, the Most Merciful
If you feel that irresolvable damages have occurred due to the negligence on the part of the doctors, then Islam acknowledges the need for remuneration and compensation for the damages incurred. Thus, after seeking legal counsel, if you feel that this is an avenue which you wish to pursue, then Shariah has no objections. [Al-Hidāyah; volume 3 page 191] 
I would also like to take this opportunity to remind myself and yourself of the following Hadith of our Beloved ﷺ, “Never a believer is stricken with a discomfort, hardship or illness, grief or even with mental worry that his sins are not expiated for him.” [Sahih Al-Muslim; Hadith 2573] 
Only Allah knows best.
Written by Maulana Mohammed Dilwar Hussain
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
فصل: “والصلح جائز عن دعوى الأموال” …قال: “ويصح عن جناية العمد والخطأ”
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنِ الْوَلِيدِ بْنِ، كَثِيرٍ عَنْ مُحَمَّدِ بْنِ عَمْرِو بْنِ عَطَاءٍ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنْ أَبِي سَعِيدٍ، وَأَبِي، هُرَيْرَةَ أَنَّهُمَا سَمِعَا رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ “ مَا يُصِيبُ الْمُؤْمِنَ مِنْ وَصَبٍ وَلاَ نَصَبٍ وَلاَ سَقَمٍ وَلاَ حَزَنٍ حَتَّى الْهَمِّ يُهَمُّهُ إِلاَّ كُفِّرَ بِهِ مِنْ سَيِّئَاتِهِ ” .