In which episodes where that woman that was pregnant and had a brain tumor and when she died Derek went dark?

I think it's in the two episodes: An honest mistake and I will follow あなた into the dark. 5x16+17

True, In 'An honest mistake' the woman actually dies and in 'I will follow あなた into the dark' Derek goes all dark with him sitting on the ソファー, ソファ all 日 and being drunk in his trailer.

but she was also in two episodes before it...she came in 5x14 - Beat your ハート, 心 out then she still was there in Before And After and dies in An Honest Mistake...